Problem:


Score : 300 points

Problem Statement

Given are N integers A_1,\ldots,A_N.

Find the sum of A_i \times A_j over all pairs (i,j) such that 1\leq i < j \leq N, modulo (10^9+7).

Constraints

2 \leq N \leq 2\times 10^5

0 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 \ldots A_N

Output

Print \sum_{i=1}^{N-1}\sum_{j=i+1}^{N} A_i A_j, modulo (10^9+7).

Sample Input 1

3
1 2 3

Sample Output 1

11

We have 1 \times 2 + 1 \times 3 + 2 \times 3 = 11.

Sample Input 2

4
141421356 17320508 22360679 244949

Sample Output 2

437235829



Sample input:
3
1 2 3


Sample output:
11

Submission status: Accepted

Solution code:
# ---------------------------iye ha aam zindegi---------------------------------------------
import math
import random
import heapq, bisect
import sys
from collections import deque, defaultdict
from fractions import Fraction
import sys
#import threading
from collections import defaultdict
#threading.stack_size(10**8)
mod = 10 ** 9 + 7
mod1 = 998244353

# ------------------------------warmup----------------------------
import os
import sys
from io import BytesIO, IOBase
sys.setrecursionlimit(300000)

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")


sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")


# -------------------game starts now----------------------------------------------------import math
class TreeNode:
    def __init__(self, k, v):
        self.key = k
        self.value = v
        self.left = None
        self.right = None
        self.parent = None
        self.height = 1
        self.num_left = 1
        self.num_total = 1


class AvlTree:

    def __init__(self):
        self._tree = None

    def add(self, k, v):
        if not self._tree:
            self._tree = TreeNode(k, v)
            return
        node = self._add(k, v)
        if node:
            self._rebalance(node)

    def _add(self, k, v):
        node = self._tree
        while node:
            if k < node.key:
                if node.left:
                    node = node.left
                else:
                    node.left = TreeNode(k, v)
                    node.left.parent = node
                    return node.left
            elif node.key < k:
                if node.right:
                    node = node.right
                else:
                    node.right = TreeNode(k, v)
                    node.right.parent = node
                    return node.right
            else:
                node.value = v
                return

    @staticmethod
    def get_height(x):
        return x.height if x else 0

    @staticmethod
    def get_num_total(x):
        return x.num_total if x else 0

    def _rebalance(self, node):

        n = node
        while n:
            lh = self.get_height(n.left)
            rh = self.get_height(n.right)
            n.height = max(lh, rh) + 1
            balance_factor = lh - rh
            n.num_total = 1 + self.get_num_total(n.left) + self.get_num_total(n.right)
            n.num_left = 1 + self.get_num_total(n.left)

            if balance_factor > 1:
                if self.get_height(n.left.left) < self.get_height(n.left.right):
                    self._rotate_left(n.left)
                self._rotate_right(n)
            elif balance_factor < -1:
                if self.get_height(n.right.right) < self.get_height(n.right.left):
                    self._rotate_right(n.right)
                self._rotate_left(n)
            else:
                n = n.parent

    def _remove_one(self, node):
        """
        Side effect!!! Changes node. Node should have exactly one child
        """
        replacement = node.left or node.right
        if node.parent:
            if AvlTree._is_left(node):
                node.parent.left = replacement
            else:
                node.parent.right = replacement
            replacement.parent = node.parent
            node.parent = None
        else:
            self._tree = replacement
            replacement.parent = None
        node.left = None
        node.right = None
        node.parent = None
        self._rebalance(replacement)

    def _remove_leaf(self, node):
        if node.parent:
            if AvlTree._is_left(node):
                node.parent.left = None
            else:
                node.parent.right = None
            self._rebalance(node.parent)
        else:
            self._tree = None
        node.parent = None
        node.left = None
        node.right = None

    def remove(self, k):
        node = self._get_node(k)
        if not node:
            return
        if AvlTree._is_leaf(node):
            self._remove_leaf(node)
            return
        if node.left and node.right:
            nxt = AvlTree._get_next(node)
            node.key = nxt.key
            node.value = nxt.value
            if self._is_leaf(nxt):
                self._remove_leaf(nxt)
            else:
                self._remove_one(nxt)
            self._rebalance(node)
        else:
            self._remove_one(node)

    def get(self, k):
        node = self._get_node(k)
        return node.value if node else -1

    def _get_node(self, k):
        if not self._tree:
            return None
        node = self._tree
        while node:
            if k < node.key:
                node = node.left
            elif node.key < k:
                node = node.right
            else:
                return node
        return None

    def get_at(self, pos):
        x = pos + 1
        node = self._tree
        while node:
            if x < node.num_left:
                node = node.left
            elif node.num_left < x:
                x -= node.num_left
                node = node.right
            else:
                return (node.key, node.value)
        raise IndexError("Out of ranges")

    @staticmethod
    def _is_left(node):
        return node.parent.left and node.parent.left == node

    @staticmethod
    def _is_leaf(node):
        return node.left is None and node.right is None

    def _rotate_right(self, node):
        if not node.parent:
            self._tree = node.left
            node.left.parent = None
        elif AvlTree._is_left(node):
            node.parent.left = node.left
            node.left.parent = node.parent
        else:
            node.parent.right = node.left
            node.left.parent = node.parent
        bk = node.left.right
        node.left.right = node
        node.parent = node.left
        node.left = bk
        if bk:
            bk.parent = node
        node.height = max(self.get_height(node.left), self.get_height(node.right)) + 1
        node.num_total = 1 + self.get_num_total(node.left) + self.get_num_total(node.right)
        node.num_left = 1 + self.get_num_total(node.left)

    def _rotate_left(self, node):
        if not node.parent:
            self._tree = node.right
            node.right.parent = None
        elif AvlTree._is_left(node):
            node.parent.left = node.right
            node.right.parent = node.parent
        else:
            node.parent.right = node.right
            node.right.parent = node.parent
        bk = node.right.left
        node.right.left = node
        node.parent = node.right
        node.right = bk
        if bk:
            bk.parent = node
        node.height = max(self.get_height(node.left), self.get_height(node.right)) + 1
        node.num_total = 1 + self.get_num_total(node.left) + self.get_num_total(node.right)
        node.num_left = 1 + self.get_num_total(node.left)

    @staticmethod
    def _get_next(node):
        if not node.right:
            return node.parent
        n = node.right
        while n.left:
            n = n.left
        return n


# -----------------------------------------------binary seacrh tree---------------------------------------
class SegmentTree:
    def __init__(self, data, default=0, func=lambda a, b: a+b):
        """initialize the segment tree with data"""
        self._default = default
        self._func = func
        self._len = len(data)
        self._size = _size = 1 << (self._len - 1).bit_length()

        self.data = [default] * (2 * _size)
        self.data[_size:_size + self._len] = data
        for i in reversed(range(_size)):
            self.data[i] = func(self.data[i + i], self.data[i + i + 1])

    def __delitem__(self, idx):
        self[idx] = self._default

    def __getitem__(self, idx):
        return self.data[idx + self._size]

    def __setitem__(self, idx, value):
        idx += self._size
        self.data[idx] = value
        idx >>= 1
        while idx:
            self.data[idx] = self._func(self.data[2 * idx], self.data[2 * idx + 1])
            idx >>= 1

    def __len__(self):
        return self._len

    def query(self, start, stop):
        if start == stop:
            return self.__getitem__(start)
        stop += 1
        start += self._size
        stop += self._size

        res = self._default
        while start < stop:
            if start & 1:
                res = self._func(res, self.data[start])
                start += 1
            if stop & 1:
                stop -= 1
                res = self._func(res, self.data[stop])
            start >>= 1
            stop >>= 1
        return res

    def __repr__(self):
        return "SegmentTree({0})".format(self.data)

# -------------------------------iye ha chutiya zindegi-------------------------------------
class Factorial:
    def __init__(self, MOD):
        self.MOD = MOD
        self.factorials = [1, 1]
        self.invModulos = [0, 1]
        self.invFactorial_ = [1, 1]

    def calc(self, n):
        if n <= -1:
            print("Invalid argument to calculate n!")
            print("n must be non-negative value. But the argument was " + str(n))
            exit()
        if n < len(self.factorials):
            return self.factorials[n]
        nextArr = [0] * (n + 1 - len(self.factorials))
        initialI = len(self.factorials)
        prev = self.factorials[-1]
        m = self.MOD
        for i in range(initialI, n + 1):
            prev = nextArr[i - initialI] = prev * i % m
        self.factorials += nextArr
        return self.factorials[n]

    def inv(self, n):
        if n <= -1:
            print("Invalid argument to calculate n^(-1)")
            print("n must be non-negative value. But the argument was " + str(n))
            exit()
        p = self.MOD
        pi = n % p
        if pi < len(self.invModulos):
            return self.invModulos[pi]
        nextArr = [0] * (n + 1 - len(self.invModulos))
        initialI = len(self.invModulos)
        for i in range(initialI, min(p, n + 1)):
            next = -self.invModulos[p % i] * (p // i) % p
            self.invModulos.append(next)
        return self.invModulos[pi]

    def invFactorial(self, n):
        if n <= -1:
            print("Invalid argument to calculate (n^(-1))!")
            print("n must be non-negative value. But the argument was " + str(n))
            exit()
        if n < len(self.invFactorial_):
            return self.invFactorial_[n]
        self.inv(n)  # To make sure already calculated n^-1
        nextArr = [0] * (n + 1 - len(self.invFactorial_))
        initialI = len(self.invFactorial_)
        prev = self.invFactorial_[-1]
        p = self.MOD
        for i in range(initialI, n + 1):
            prev = nextArr[i - initialI] = (prev * self.invModulos[i % p]) % p
        self.invFactorial_ += nextArr
        return self.invFactorial_[n]


class Combination:
    def __init__(self, MOD):
        self.MOD = MOD
        self.factorial = Factorial(MOD)

    def ncr(self, n, k):
        if k < 0 or n < k:
            return 0
        k = min(k, n - k)
        f = self.factorial
        return f.calc(n) * f.invFactorial(max(n - k, k)) * f.invFactorial(min(k, n - k)) % self.MOD


# --------------------------------------iye ha combinations ka zindegi---------------------------------
def powm(a, n, m):
    if a == 1 or n == 0:
        return 1
    if n % 2 == 0:
        s = powm(a, n // 2, m)
        return s * s % m
    else:
        return a * powm(a, n - 1, m) % m


# --------------------------------------iye ha power ka zindegi---------------------------------
def sort_list(list1, list2):
    zipped_pairs = zip(list2, list1)

    z = [x for _, x in sorted(zipped_pairs)]

    return z


# --------------------------------------------------product----------------------------------------
def product(l):
    por = 1
    for i in range(len(l)):
        por *= l[i]
    return por


# --------------------------------------------------binary----------------------------------------
def binarySearchCount(arr, n, key):
    left = 0
    right = n - 1

    count = 0

    while (left <= right):
        mid = int((right + left) / 2)

        # Check if middle element is
        # less than or equal to key
        if (arr[mid] <= key):
            count = mid + 1
            left = mid + 1

        # If key is smaller, ignore right half
        else:
            right = mid - 1

    return count


# --------------------------------------------------binary----------------------------------------
def countdig(n):
    c = 0
    while (n > 0):
        n //= 10
        c += 1
    return c
def binary(x, length):
    y = bin(x)[2:]
    return y if len(y) >= length else "0" * (length - len(y)) + y

def countGreater(arr, n, k):
    l = 0
    r = n - 1

    # Stores the index of the left most element
    # from the array which is greater than k
    leftGreater = n

    # Finds number of elements greater than k
    while (l <= r):
        m = int(l + (r - l) / 2)
        if (arr[m] >= k):
            leftGreater = m
            r = m - 1

        # If mid element is less than
        # or equal to k update l
        else:
            l = m + 1

    # Return the count of elements
    # greater than k
    return (n - leftGreater)


# --------------------------------------------------binary------------------------------------
n=int(input())
l=list(map(int,input().split()))
su=l[0]
ans=0
for i in range(1,n):
    ans+=su*l[i]
    ans%=mod
    su+=l[i]
    su%=mod
print(ans)

Submission status: Accepted

Solution code:
n = int(input())
a = list(map(int,input().split()))
p = 0
q = a[0]
for i in range(1,n):
    p += q*a[i]
    p %= 1000000007
    q += a[i]
    q %= 1000000007
print(p)

Submission status: Accepted

Solution code:
def main():
    import sys
    sys.setrecursionlimit(10**6)
    input = sys.stdin.readline
    N = int(input())
    A = [int(x) for x in input().strip().split()]
    MOD = 10 ** 9 + 7
    sumA = sum(A)
    ans = 0
    for n in range(N-1):
        sumA -= A[n]
        ans += sumA * A[n]
        ans %= MOD
    print(ans)

if __name__ == '__main__':
    main()

Submission status: Accepted

Solution code:
mod=10**9+7
n=int(input())
a=list(map(int,input().split()))
cnt=0
cnt2=0
for x in a:
	cnt+=x**2
	cnt2+=x
	cnt%=mod
	cnt2%=mod
ans=cnt2**2%mod-cnt
if ans%2==1:
	ans+=mod
ans=ans//2%mod
print(ans)

Submission status: Accepted

Solution code:
n = int(input())
a = list(map(int, input().split()))

s = sum(a)
ans = 0
for i in a:
    s -= i
    ans += i * s
    ans %= 10 ** 9 + 7
print(ans)

Submission status: Accepted

Solution code:
mod=10**9+7
n=int(input())
a=list(map(int,input().split()))
s=a[-1]
c=0
for i in range (n-2,-1,-1):
    c+=a[i]*s
    c%=mod
    s+=a[i]
print(c)

Submission status: Accepted

Solution code:
#!/usr/bin/env python3
import os
from sys import stdin, stdout
    
    
def solve(tc):
    MOD = int(1e9) + 7
    n = int(stdin.readline().strip())
    seq = list(map(int, stdin.readline().split()))

    suffix = [0 for i in range(n)]
    suffix[n-1] = seq[n-1]
    for i in range(n-2, -1, -1):
        suffix[i] = suffix[i+1] + seq[i]
    
    ans = 0
    for i in range(n-1):
        ans += seq[i] * suffix[i+1]
        ans %= MOD
    
    print(ans)


    
tcs = 1
tc = 1
while tc <= tcs:
    solve(tc)
    tc += 1

Submission status: Accepted

Solution code:
#!/usr/bin/env python
#pyrival orz
import os
import sys
from io import BytesIO, IOBase

"""
    for _ in range(int(input())):
    n,m=map(int,input().split())
    n=int(input())
    a = [int(x) for x in input().split()]
"""

def main():
    mod=10**9+7
    ans=0
    s=0
    n=int(input())
    a = [int(x) for x in input().split()]
    for x in a:
        ans+=s*x
        ans%=mod
        s+=x
        s%=mod
    print(ans)


# region fastio

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")


sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")

# endregion

if __name__ == "__main__":
    main()

Submission status: Accepted

Solution code:
MOD = 10**9 + 7
import sys
from itertools import accumulate
input = sys.stdin.readline
N = int(input())
A = list(map(int,input().split()))

cs = accumulate(A)

ans = 0
for s,t in zip(cs, A[1:]):
    ans += s * t
    ans %= MOD

print(ans)

Submission status: Accepted

Solution code:
n = int(input())
a = list(map(int,input().split()))
MOD = 10**9 + 7

rui = [0]*n
cur = 0
for i in range(n):
    cur = (cur+a[i]) % MOD
    rui[i] = cur
    
ans = 0
for i in range(n-1):
    ans = (ans + (a[i+1] * rui[i])) % MOD
    
print(ans)

Submission status: Accepted

Solution code:
import sys
readline = sys.stdin.buffer.readline
def even(n): return 1 if n%2==0 else 0

n = int(readline())
lst1 = list(map(int,readline().split()))
mod = 10**9+7

res = 0

ans = 0
al = sum(lst1)
for i in lst1:
    res += i
    ans += (al-res)*i
    ans %= mod

    

print(ans)

Submission status: Accepted

Solution code:
N = int(input())
A = list(map(int, input().split()))
MOD = 10 ** 9 + 7

S = sum(A)
S %= MOD
ans = 0
for a in A:
    S += MOD
    S -= a
    S %= MOD
    ans += a * S
    ans %= MOD

print(ans)

Submission status: Accepted

Solution code:


mod=10**9+7

sm=0

n=int(input())
a=list(map(int,input().split()))

ans=0

for i in a:
	sm+=i
	sm%=mod

for i in a:
	sm-=i
	sm+=mod
	sm%=mod
	ans+=(i*sm)%mod
	ans%=mod

print(ans)

Submission status: Accepted

Solution code:
N = int(input())
nums = list(map(int, input().split()))
print((sum(nums)**2 - sum([i**2 for i in nums])) // 2 % (10**9 + 7))

Submission status: Accepted

Solution code:
MOD = 10 ** 9 + 7
n = int(input())
a = list(map(int, input().split()))
b = [0] * n
for i in range(n - 1):
    b[-i - 2] = b[-i-1] + a[-i-1]
ans = 0
for i in range(n - 1):
    ans += a[i] * b[i]
print(ans % MOD)

Submission status: Accepted

Solution code:
from math import inf, log2


class SegmentTree:
    def __init__(self, array, func=max):
        self.n = len(array)
        self.size = 2**(int(log2(self.n-1))+1) if self.n != 1 else 1
        self.func = func
        self.default = 0 if self.func != min else inf
        self.data = [self.default] * (2 * self.size)
        self.process(array)

    def process(self, array):
        self.data[self.size : self.size+self.n] = array
        for i in range(self.size-1, -1, -1):
            self.data[i] = self.func(self.data[2*i], self.data[2*i+1])

    def query(self, alpha, omega):
        """Returns the result of function over the range (inclusive)!"""
        if alpha == omega:
            return self.data[alpha + self.size]
        res = self.default
        alpha += self.size
        omega += self.size + 1
        while alpha < omega:
            if alpha & 1:
                res = self.func(res, self.data[alpha])
                alpha += 1
            if omega & 1:
                omega -= 1
                res = self.func(res, self.data[omega])
            alpha >>= 1
            omega >>= 1
        return res

    def update(self, index, value):
        """Updates the element at index to given value!"""
        index += self.size
        self.data[index] = value
        index >>= 1
        while index:
            self.data[index] = self.func(self.data[2*index], self.data[2*index+1])
            index >>= 1


# ------------------- fast io --------------------
import os
import sys
from io import BytesIO, IOBase

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")


sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")

# ------------------- fast io --------------------
from math import gcd, ceil

def pre(s):
    n = len(s)
    pi = [0] * n
    for i in range(1, n):
        j = pi[i - 1]
        while j and s[i] != s[j]:
            j = pi[j - 1]
        if s[i] == s[j]:
            j += 1
        pi[i] = j
    return pi


def prod(a):
    ans = 1
    for each in a:
        ans = (ans * each)
    return ans

def lcm(a, b): return a * b // gcd(a, b)


def binary(x, length=16):
    y = bin(x)[2:]
    return y if len(y) >= length else "0" * (length - len(y)) + y


for _ in range(int(input()) if not True else 1):
    n = int(input())
    #d, t, s = map(int, input().split())
    #a, b = map(int, input().split())
    #c, d = map(int, input().split())
    a = list(map(int, input().split()))
    #b = list(map(int, input().split()))
    st = SegmentTree(a, func=lambda a,b:a+b)
    ans=0
    for i in range(n-1):
        ans = (ans + a[i] * st.query(i+1, n-1))  % (10**9 + 7)
    print(ans)

Submission status: Accepted

Solution code:
n=int(input())
a=list(map(int,input().split()))
x=sum(a)
y=sum([(a[i])**2 for i in range(n)])
print(((x**2-y)//2)%(10**9+7))

Submission status: Accepted

Solution code:
import sys

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

in_n = lambda: int(readline())
in_nn = lambda: map(int, readline().split())
in_s = lambda: readline().rstrip().decode('utf-8')
in_nl = lambda: list(map(int, readline().split()))
in_nl2 = lambda H: [in_nl() for _ in range(H)]
in_map = lambda: [s == ord('.') for s in readline() if s != ord('\n')]
in_map2 = lambda H: [in_map() for _ in range(H)]
in_all = lambda: map(int, read().split())


def main():

    N = in_n()
    A = in_nl()

    ans = 0
    mod = 10**9 + 7
    sum_a = sum(A)
    tsum = [0] * (N + 1)

    for i in range(N):
        tsum[i + 1] = tsum[i] + A[i]

    for i in range(N):
        ans += A[i] * (sum_a - tsum[i + 1])
        ans %= mod

    print(ans)


if __name__ == '__main__':
    main()

Submission status: Accepted

Solution code:
N = int(input())
A = list(map(int,input().split()))

B = [0]
b = 0

mod = 10**9 + 7

for i in range(N):
    b+=A[i]
    b%=mod
    B.append(b)

ans = 0

for i in range(N):
    ans += (mod+B[N]-B[i+1])*A[i]
    ans %= mod

print(ans)

Submission status: Accepted

Solution code:
import sys, math
import io, os
#data = io.BytesIO(os.read(0,os.fstat(0).st_size)).readline
from bisect import bisect_left as bl, bisect_right as br, insort
from heapq import heapify, heappush, heappop
from collections import defaultdict as dd, deque, Counter
# from itertools import permutations,combinations
def data(): return sys.stdin.readline().strip()
def mdata(): return list(map(int, data().split()))
def outl(var): sys.stdout.write(' '.join(map(str, var)) + '\n')
def out(var): sys.stdout.write(str(var) + '\n')
from decimal import Decimal
# from fractions import Fraction
# sys.setrecursionlimit(100000)
INF = float('inf')
mod = int(1e9) + 7


n=int(data())
a=mdata()
s=[0]*n
s[-1]=a[-1]
for i in range(n-2,-1,-1):
    s[i]=s[i+1]+a[i]
ans=0
for i in range(n-1):
    ans=(ans+a[i]*s[i+1])%mod
out(ans)

Submission status: Accepted

Solution code:
from sys import stdin, setrecursionlimit
input = stdin.buffer.readline
setrecursionlimit(10 ** 7)

from heapq import heappush, heappop
from bisect import bisect_left, bisect_right
from collections import deque, defaultdict, Counter
from itertools import combinations, permutations, combinations_with_replacement
from itertools import accumulate
from math import ceil, sqrt, pi, radians, sin, cos

MOD = 10 ** 9 + 7
INF = 10 ** 18

N = int(input())
#N, M = map(int, input().split())
A = list(map(int, input().split()))
#A = [int(input()) for _ in range(N)]
#ABC = [list(map(int, input().split())) for _ in range(N)]
#S = input()
#S = [input() for _ in range(N)]

acum_A = 0
for a in A:
    acum_A += a

answer = 0
for a in A:
    acum_A -= a
    answer = (answer + a * (acum_A)) % MOD

print(answer)

Submission status: Accepted

Solution code:
n=int(input())
a=list(map(int,input().split()))
s=sum(a)
ans=0
mod=10**9+7
for i in a:
    ans=(ans+i*(s-i))
print((ans//2)%mod)

Submission status: Accepted

Solution code:
n=int(input())
A=list(map(int,input().split()))
mod=10**9+7
sum_a=sum(A)
ans=0
for a in A:
    ans += a*(sum_a-a)
print((ans//2)%mod)

Submission status: Accepted

Solution code:
import sys
input = sys.stdin.readline
from collections import *

N = int(input())
A = list(map(int, input().split()))
MOD = 10**9+7
ans = 0
acc = 0

for i in range(N):
    ans += A[i]*acc
    ans %= MOD
    acc += A[i]
    acc %= MOD

print(ans)

Submission status: Accepted

Solution code:

N = int(input())
X = list(map(int, input().split()))
MOD = 10 ** 9 + 7

cnt = [0] * (N + 1)
for i in reversed(range(N)):
    cnt[i] = cnt[i + 1] + X[i] % MOD

res = 0
for i in range(N):
    res += X[i] * cnt[i + 1] % MOD
    res %= MOD
print(res)

Submission status: Accepted

Solution code:
MOD = 10**9+7

n = int(input())
a = list(map(int,input().split()))

asum = sum(a) % MOD
asq  = sum(x*x %MOD for x in a) %MOD

ans = (asum*asum - asq) %MOD * pow(2,MOD-2,MOD) %MOD
print(ans)

Submission status: Accepted

Solution code:
import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def SI(): return sys.stdin.readline()[:-1]

md=10**9+7
n=II()
aa=LI()
s=pow(sum(aa),2,md)
for a in aa:
    s-=a**2
    s%=md
inv2=pow(2,md-2,md)
s=s*inv2%md
print(s)

Submission status: Accepted

Solution code:
#!/usr/bin/env python3
import sys
input = sys.stdin.readline
MOD = 10**9 + 7
 
n = int(input())
a = [int(item) for item in input().split()]
total = sum(a)

ans = 0
for item in a:
    total -= item
    ans += total * item
    ans %= MOD

print(ans)

Submission status: Accepted

Solution code:
N=int(input())
A=list(map(int,input().split()))
Mod=10**9+7
two_inverse=pow(2,Mod-2,Mod)

X=sum(A)%Mod

Y=0
for a in A:
    Y+=a*a
    Y%=Mod

print(((X*X-Y)*two_inverse)%Mod)

Submission status: Accepted

Solution code:
import sys
from itertools import accumulate

read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines
sys.setrecursionlimit(10 ** 9)
INF = 1 << 60
MOD = 1000000007


def main():
    N, *A = map(int, read().split())

    csum = [0]
    csum.extend(accumulate(reversed(A)))

    ans = 0
    for i, a in enumerate(A):
        ans = (ans + a * csum[N - i - 1] % MOD) % MOD

    print(ans)
    return


if __name__ == '__main__':
    main()

Submission status: Accepted

Solution code:
import sys
input = sys.stdin.readline

N=int(input())
A=list(map(int,input().split()))

S=sum(A)
ANS=0
mod=10**9+7
for i in range(N):
    ANS=ANS+(S-A[i])*A[i]
    ANS%=mod

print(ANS*pow(2,-1,mod)%mod)