Problem:


Score: 500 points

Problem Statement

Given is a permutation P of \{1, 2, \ldots, N\}.

For a pair (L, R) (1 \le L \lt R \le N), let X_{L, R} be the second largest value among P_L, P_{L+1}, \ldots, P_R.

Find \displaystyle \sum_{L=1}^{N-1} \sum_{R=L+1}^{N} X_{L,R}.

Constraints

 2 \le N \le 10^5 

 1 \le P_i \le N 

 P_i \neq P_j  (i \neq j)

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
P_1 P_2 \ldots P_N

Output

Print \displaystyle \sum_{L=1}^{N-1} \sum_{R=L+1}^{N} X_{L,R}.

Sample Input 1

3
2 3 1

Sample Output 1

5

X_{1, 2} = 2, X_{1, 3} = 2, and X_{2, 3} = 1, so the sum is 2 + 2 + 1 = 5.

Sample Input 2

5
1 2 3 4 5

Sample Output 2

30

Sample Input 3

8
8 2 7 3 4 5 6 1

Sample Output 3

136



Sample input:
3
2 3 1


Sample output:
5

Submission status: Accepted

Solution code:
from bisect import bisect_left, bisect_right, insort_right
class SquareSkipList:
    # SkipList の層数を 2 にした感じの何か
    # std::multiset の代用になる
    def __init__(self, values=None, sorted_=False, square=1000, seed=42):
        # values: 初期値のリスト
        # sorted_: 初期値がソート済みであるか
        # square: 最大データ数の平方根
        # seed: 乱数のシード
        inf = float("inf")
        self.rand_y = seed
        self.square = square
        if values is None:
            self.layer1 = [inf]
            self.layer0 = {inf: []}
        else:
            self.layer1 = layer1 = []
            if not sorted_:
                values.sort()
            rand_depth = self._rand_depth
            l0 = []
            layer0 = []
            for v in values:
                if rand_depth():
                    layer0.append(l0)
                    l0 = []
                    layer1.append(v)
                else:
                    l0.append(v)
            layer0.append(l0)
            self.layer0 = dict(zip([inf]+layer1, layer0))
            layer1.append(inf)

    def _rand_depth(self):  # 32bit xorshift
        y = self.rand_y
        y ^= y << 13 & 0xffffffff
        y ^= y >> 17
        y ^= y << 5 & 0xffffffff
        self.rand_y = y
        return y % self.square == 0

    def add(self, x):  # 要素の追加  # O(sqrt(n))
        layer1, layer0 = self.layer1, self.layer0
        if self._rand_depth():
            idx1 = bisect_right(layer1, x)
            val1 = layer1[idx1-1]
            layer1.insert(idx1, x)
            layer0_idx1 = layer0[val1]
            idx0 = bisect_right(layer0_idx1, x)
            layer0[x] = layer0_idx1[idx0:]
            del layer0_idx1[idx0:]
        else:
            idx1 = bisect_right(layer1, x)
            val1 = layer1[idx1-1]
            insort_right(layer0[val1], x)

    def remove(self, x):  # 要素の削除  # O(sqrt(n))
        # x が存在しない場合、x 以上の最小の要素が削除される
        raise NotImplementedError
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            del layer1[idx1]
            layer0[idx1] += layer0.pop(idx1+1)
        else:
            del layer0_idx1[idx0]

    def search_higher_equal(self, x):  # x 以上の最小の値を返す  O(log(n))
        raise NotImplementedError
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            return layer1[idx1]
        return layer0_idx1[idx0]

    def search_higher(self, x):  # x を超える最小の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_right(layer1, x)
        val1 = layer1[idx1-1]
        layer0_idx1 = layer0[val1]
        idx0 = bisect_right(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            return layer1[idx1]
        return layer0_idx1[idx0]

    def search_lower(self, x):  # x 未満の最大の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        val1 = layer1[idx1-1]
        layer0_idx1 = layer0[val1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == 0:  # layer0_idx1 が空の場合とすべて x 以上の場合
            return layer1[idx1-1]
        return layer0_idx1[idx0-1]

    def pop(self, idx):
        raise NotImplementedError
        # 小さい方から idx 番目の要素を削除してその要素を返す（0-indexed）
        # O(sqrt(n))
        # for を回すので重め、使うなら square パラメータを大きめにするべき
        layer1, layer0 = self.layer1, self.layer0
        s = -1
        for i, l0 in enumerate(layer0):
            s += len(l0) + 1
            if s >= idx:
                break
        if s==idx:
            layer0[i] += layer0[i+1]
            del layer0[i+1]
            return layer1.pop(i)
        else:
            return layer0[i].pop(idx-s)

    def print(self):
        print(self.layer1)
        print(*sorted(list(self.layer0.items())), sep="\n")


def main():
    # 参考: https://atcoder.jp/contests/abc140/submissions/7477790
    n = int(input())
    p = list(map(int, input().split()))
    idx = [0] * n
    for i in range(0, n):
        idx[i] = i
    idx.sort(key=lambda i: - p[i])
    ssl = SquareSkipList(square=int(n**0.5))
    ssl.add(-1)
    ssl.add(n)
    ans = 0
    for i in idx:
        nex = ssl.search_higher(i)
        nexnex = ssl.search_higher(nex)
        pre = ssl.search_lower(i)
        prepre = ssl.search_lower(pre)
        if prepre != float("inf"):
            ans += p[i] * (pre - prepre) * (nex - i)
        if nexnex != float("inf"):
            ans += p[i] * (i - pre) * (nexnex - nex)
        ssl.add(i)
    print(ans)


main()

Submission status: Accepted

Solution code:
from bisect import bisect_left, bisect_right, insort_right
class SquareSkipList:
    # SkipList の層数を 2 にした感じの何か
    # std::multiset の代用になる
    # 検証1 (add, pop) データ構造: https://atcoder.jp/contests/arc033/submissions/7480578
    # 検証2 (init, add, remove, search_higher_equal) Exclusive OR Queries: https://atcoder.jp/contests/cpsco2019-s1/submissions/7484086
    # 検証3 (add, search_higher, search_lower) Second Sum: https://atcoder.jp/contests/abc140/submissions/7483678
    def __init__(self, values=None, sorted_=False, square=1000, seed=42):
        # values: 初期値のリスト
        # sorted_: 初期値がソート済みであるか
        # square: 最大データ数の平方根
        # seed: 乱数のシード
        inf = float("inf")
        self.rand_y = seed
        self.square = square
        if values is None:
            self.layer1 = []
            self.layer0 = [[]]
        else:
            self.layer1 = layer1 = []
            self.layer0 = layer0 = []
            if not sorted_:
                values.sort()
            rand_depth = self._rand_depth
            l0 = []
            for v in values:
                if rand_depth():
                    layer0.append(l0)
                    l0 = []
                    layer1.append(v)
                else:
                    l0.append(v)
            layer0.append(l0)
        self.layer1.append(inf)

    def _rand_depth(self):  # 32bit xorshift
        y = self.rand_y
        y ^= y << 13 & 0xffffffff
        y ^= y >> 17
        y ^= y << 5 & 0xffffffff
        self.rand_y = y
        return y % self.square == 0

    def add(self, x):  # 要素の追加  # O(sqrt(n))
        layer1, layer0 = self.layer1, self.layer0
        if self._rand_depth():
            idx1 = bisect_right(layer1, x)
            layer1.insert(idx1, x)
            layer0_idx1 = layer0[idx1]
            idx0 = bisect_right(layer0_idx1, x)
            layer0.insert(idx1+1, layer0_idx1[idx0:])  # layer0 は dict で管理した方が良いかもしれない
            del layer0_idx1[idx0:]
        else:
            idx1 = bisect_right(layer1, x)
            insort_right(layer0[idx1], x)

    def remove(self, x):  # 要素の削除  # O(sqrt(n))
        # x が存在しない場合、x 以上の最小の要素が削除される
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            del layer1[idx1]
            layer0[idx1] += layer0.pop(idx1+1)
        else:
            del layer0_idx1[idx0]

    def search_higher_equal(self, x):  # x 以上の最小の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            return layer1[idx1]
        return layer0_idx1[idx0]

    def search_higher(self, x):  # x を超える最小の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_right(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_right(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            return layer1[idx1]
        return layer0_idx1[idx0]

    def search_lower(self, x):  # x 未満の最大の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == 0:  # layer0_idx1 が空の場合とすべて x 以上の場合
            return layer1[idx1-1]
        return layer0_idx1[idx0-1]

    def pop(self, idx):
        # 小さい方から idx 番目の要素を削除してその要素を返す（0-indexed）
        # O(sqrt(n))
        # for を回すので重め、使うなら square パラメータを大きめにするべき
        layer1, layer0 = self.layer1, self.layer0
        s = -1
        for i, l0 in enumerate(layer0):
            s += len(l0) + 1
            if s >= idx:
                break
        if s==idx:
            layer0[i] += layer0[i+1]
            del layer0[i+1]
            return layer1.pop(i)
        else:
            return layer0[i].pop(idx-s)

    def print(self):
        print(self.layer1)
        print(self.layer0)


def main():
    # 参考: https://atcoder.jp/contests/abc140/submissions/7477790
    n = int(input())
    p = list(map(int, input().split()))
    idx = [0] * n
    for i in range(0, n):
        idx[i] = i
    idx.sort(key=lambda i: - p[i])
    ssl = SquareSkipList(square=int(n**0.5))
    ssl.add(-1)
    ssl.add(n)
    ans = 0
    for i in idx:
        nex = ssl.search_higher(i)
        nexnex = ssl.search_higher(nex)
        pre = ssl.search_lower(i)
        prepre = ssl.search_lower(pre)
        if prepre != float("inf"):
            ans += p[i] * (pre - prepre) * (nex - i)
        if nexnex != float("inf"):
            ans += p[i] * (i - pre) * (nexnex - nex)
        ssl.add(i)
    print(ans)


main()

Submission status: Accepted

Solution code:
from bisect import bisect_left, bisect_right, insort_right
class SquareSkipList:
    # SkipList の層数を 2 にした感じの何か
    # std::multiset の代用になる
    # 検証1 (add, pop) データ構造: https://atcoder.jp/contests/arc033/submissions/7480578
    # 検証2 (init, add, remove, search_higher_equal) Exclusive OR Queries: https://atcoder.jp/contests/cpsco2019-s1/submissions/7484086
    # 検証3 (add, search_higher, search_lower) Second Sum: https://atcoder.jp/contests/abc140/submissions/7483678
    def __init__(self, values=None, sorted_=False, square=1000, seed=42):
        # values: 初期値のリスト
        # sorted_: 初期値がソート済みであるか
        # square: 最大データ数の平方根
        # seed: 乱数のシード
        inf = float("inf")
        self.rand_y = seed
        self.square = square
        if values is None:
            self.layer1 = []
            self.layer0 = [[]]
        else:
            self.layer1 = layer1 = []
            self.layer0 = layer0 = []
            if not sorted_:
                values.sort()
            rand_depth = self._rand_depth
            l0 = []
            for v in values:
                if rand_depth():
                    layer0.append(l0)
                    l0 = []
                    layer1.append(v)
                else:
                    l0.append(v)
            layer0.append(l0)
        self.layer1.append(inf)

    def _rand_depth(self):  # 32bit xorshift
        y = self.rand_y
        y ^= y << 13 & 0xffffffff
        y ^= y >> 17
        y ^= y << 5 & 0xffffffff
        self.rand_y = y
        return y % self.square == 0

    def add(self, x):  # 要素の追加  # O(sqrt(n))
        layer1, layer0 = self.layer1, self.layer0
        if self._rand_depth():
            idx1 = bisect_right(layer1, x)
            layer1.insert(idx1, x)
            layer0_idx1 = layer0[idx1]
            idx0 = bisect_right(layer0_idx1, x)
            layer0.insert(idx1+1, layer0_idx1[idx0:])  # layer0 は dict で管理した方が良いかもしれない
            del layer0_idx1[idx0:]
        else:
            idx1 = bisect_right(layer1, x)
            insort_right(layer0[idx1], x)

    def remove(self, x):  # 要素の削除  # O(sqrt(n))
        # x が存在しない場合、x 以上の最小の要素が削除される
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            del layer1[idx1]
            layer0[idx1] += layer0.pop(idx1+1)
        else:
            del layer0_idx1[idx0]

    def search_higher_equal(self, x):  # x 以上の最小の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            return layer1[idx1]
        return layer0_idx1[idx0]

    def search_higher(self, x):  # x を超える最小の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_right(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_right(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            return layer1[idx1]
        return layer0_idx1[idx0]

    def search_lower(self, x):  # x 未満の最大の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == 0:  # layer0_idx1 が空の場合とすべて x 以上の場合
            return layer1[idx1-1]
        return layer0_idx1[idx0-1]

    def pop(self, idx):
        # 小さい方から idx 番目の要素を削除してその要素を返す（0-indexed）
        # O(sqrt(n))
        # for を回すので重め、使うなら square パラメータを大きめにするべき
        layer1, layer0 = self.layer1, self.layer0
        s = -1
        for i, l0 in enumerate(layer0):
            s += len(l0) + 1
            if s >= idx:
                break
        if s==idx:
            layer0[i] += layer0[i+1]
            del layer0[i+1]
            return layer1.pop(i)
        else:
            return layer0[i].pop(idx-s)

    def print(self):
        print(self.layer1)
        print(self.layer0)


def main():
    # 参考: https://atcoder.jp/contests/abc140/submissions/7477790
    n = int(input())
    p = list(map(int, input().split()))
    idx = [0] * n
    for i in range(0, n):
        idx[i] = i
    idx.sort(key=lambda i: - p[i])
    ssl = SquareSkipList()
    ssl.add(-1)
    ssl.add(n)
    ans = 0
    for i in idx:
        nex = ssl.search_higher(i)
        nexnex = ssl.search_higher(nex)
        pre = ssl.search_lower(i)
        prepre = ssl.search_lower(pre)
        if prepre != float("inf"):
            ans += p[i] * (pre - prepre) * (nex - i)
        if nexnex != float("inf"):
            ans += p[i] * (i - pre) * (nexnex - nex)
        ssl.add(i)
    print(ans)


main()

Submission status: Accepted

Solution code:
from bisect import bisect_left, bisect_right, insort_right
class SquareSkipList:
    # SkipList の層数を 2 にした感じの何か
    # std::multiset の代用になる
    # 検証1 (データ構造): https://atcoder.jp/contests/arc033/submissions/7480578
    # 検証2 (Exclusive OR Queries): https://atcoder.jp/contests/cpsco2019-s1/submissions/7482199
    # 検証3 (Second Sum): https://atcoder.jp/contests/abc140/submissions/7482046
    def __init__(self, values=None, sorted_=False, square=1000, seed=42):
        # values: 初期値のリスト
        # sorted_: 初期値がソート済みであるか
        # square: 最大データ数の平方根
        # seed: 乱数のシード
        inf = float("inf")
        self.rand_y = seed
        self.square = square
        if values is None:
            self.layer1 = []
            self.layer0 = [[]]
        else:
            self.layer1 = layer1 = []
            self.layer0 = layer0 = []
            if not sorted_:
                values.sort()
            rand_depth = self.rand_depth
            l0 = []
            for v in values:
                if rand_depth():
                    layer0.append(l0)
                    l0 = []
                    layer1.append(v)
                else:
                    l0.append(v)
            layer0.append(l0)
        self.layer1.append(inf)

    def rand_depth(self):  # 32bit xorshift
        y = self.rand_y
        y ^= y << 13 & 0xffffffff
        y ^= y >> 17
        y ^= y << 5 & 0xffffffff
        self.rand_y = y
        return y % self.square == 0

    def add(self, x):  # 要素の追加  # O(sqrt(n))
        layer1, layer0 = self.layer1, self.layer0
        if self.rand_depth():
            idx1 = bisect_right(layer1, x)
            layer1.insert(idx1, x)
            layer0_idx1 = layer0[idx1]
            idx0 = bisect_right(layer0_idx1, x)
            layer0.insert(idx1+1, layer0_idx1[idx0:])  # layer0 は dict で管理した方が良いかもしれない
            del layer0_idx1[idx0:]
        else:
            idx1 = bisect_right(layer1, x)
            insort_right(layer0[idx1], x)

    def remove(self, x):  # 要素の削除  # O(sqrt(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        if layer1[idx1] == x:
            del layer1[idx1]
            layer0[idx1] += layer0[idx1+1]
            del layer0[idx1+1]
        else:
            layer0_idx1 = layer0[idx1]
            del layer0_idx1[bisect_left(layer0_idx1, x)]

    def bisect_left(self, x):  # x 以上の最小の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        res = layer1[idx1]
        if res == x:
            return res
        layer0_idx1 = layer0[idx1]
        if layer0_idx1:
            idx0 = bisect_left(layer0_idx1, x)
            if idx0 == len(layer0_idx1):
                return res
            else:
                return layer0_idx1[idx0]
        else:
            return res

    def search_higher(self, x):  # x を超える最小の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_right(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_right(layer0_idx1, x)
        if idx0 == len(layer0_idx1):
            return layer1[idx1]
        return layer0_idx1[idx0]

    def search_lower(self, x):  # x 未満の最大の値を返す  O(log(n))
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        layer0_idx1 = layer0[idx1]
        idx0 = bisect_left(layer0_idx1, x)
        if idx0 == 0:  # layer0_idx1 が空の場合とすべて x 以上の場合
            return layer1[idx1-1]
        return layer0_idx1[idx0-1]

    def pop(self, idx):
        # 小さい方から idx 番目の要素を削除してその要素を返す（0-indexed）
        # O(sqrt(n))
        # for を回すので重め  使うなら square パラメータを大きめにするべき
        layer1, layer0 = self.layer1, self.layer0
        s = -1
        for i, l0 in enumerate(layer0):
            s += len(l0) + 1
            if s >= idx:
                break
        if s==idx:
            layer0[i] += layer0[i+1]
            del layer0[i+1]
            return layer1.pop(i)
        else:
            return layer0[i].pop(idx-s)

    def print(self):
        print(self.layer1)
        print(self.layer0)



def main():
    # 参考: https://atcoder.jp/contests/abc140/submissions/7477790
    n = int(input())
    p = list(map(int, input().split()))
    idx = [0] * n
    for i in range(0, n):
        idx[i] = i
    idx.sort(key=lambda i: - p[i])
    ssl = SquareSkipList()
    ssl.add(-1)
    ssl.add(n)
    ans = 0
    for i in idx:
        nex = ssl.search_higher(i)
        nexnex = ssl.search_higher(nex)
        pre = ssl.search_lower(i)
        prepre = ssl.search_lower(pre)
        if prepre != float("inf"):
            ans += p[i] * (pre - prepre) * (nex - i)
        if nexnex != float("inf"):
            ans += p[i] * (i - pre) * (nexnex - nex)
        ssl.add(i)
    print(ans)


main()

Submission status: Accepted

Solution code:
from bisect import bisect_left, bisect_right, insort_right
class CubeSkipList:
    # SkipList の層数を 3 にした感じの何か
    # std::multiset の代用になる
    def __init__(self, values=None, sorted_=False, cube=100, seed=42):
        # values: 初期値のリスト
        # sorted_: 初期値がソート済みであるか
        # square: 最大データ数の平方根
        # seed: 乱数のシード
        inf = float("inf")
        self.rand_y = seed
        self.cube = cube
        if values is None:
            self.layer2 = [inf]
            self.layer1 = [[]]
            self.layer0 = [[[]]]
        else:
            self.layer2 = layer2 = []
            self.layer1 = layer1 = []
            self.layer0 = layer0 = []
            if not sorted_:
                values.sort()
            rand_depth = self.rand_depth
            l1 = []
            la0 = []
            l0 = []
            for v in values:
                r = rand_depth()
                if r==0:
                    l0.append(v)
                elif r==1:
                    la0.append(l0)
                    l0 = []
                    l1.append(v)
                else:
                    la0.append(l0)
                    layer0.append(la0)
                    layer1.append(l1)
                    l0 = []
                    la0 = []
                    l1 = []
                    layer2.append(v)
            la0.append(l0)
            layer0.append(la0)
            layer1.append(l1)
            layer2.append(inf)

    def rand_depth(self):  # 32bit xorshift
        y = self.rand_y
        y ^= y << 13 & 0xffffffff
        y ^= y >> 17
        y ^= y << 5 & 0xffffffff
        self.rand_y = y
        if y % self.cube == 0:
            if y % (self.cube ** 2) == 0:
                return 2
            return 1
        return 0

    def add(self, x):  # 要素の追加  # O(cbrt(n))
        layer2, layer1, layer0 = self.layer2, self.layer1, self.layer0
        r = self.rand_depth()
        if r==0:
            idx2 = bisect_right(layer2, x)
            idx1 = bisect_right(layer1[idx2], x)
            insort_right(layer0[idx2][idx1], x)
        elif r==1:
            idx2 = bisect_right(layer2, x)
            l1 = layer1[idx2]
            idx1 = bisect_right(l1, x)
            l1.insert(idx1, x)
            la0 = layer0[idx2]
            l0 = la0[idx1]
            idx0 = bisect_right(l0, x)

            la0.insert(idx1+1, l0[idx0:])
            del l0[idx0:]
        else:
            idx2 = bisect_right(layer2, x)
            layer2.insert(idx2, x)
            l1 = layer1[idx2]
            idx1 = bisect_right(l1, x)
            la0 = layer0[idx2]
            l0 = la0[idx1]
            idx0 = bisect_right(l0, x)

            la0.insert(idx1+1, l0[idx0:])
            del l0[idx0:]
            layer0.insert(idx2+1, la0[idx1+1:])
            del la0[idx1+1:]
            layer1.insert(idx2+1, l1[idx1:])
            del l1[idx1:]

    def remove(self, x):  # 要素の削除  # O(cbrt(n))
        raise NotImplementedError
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        if layer1[idx1] == x:
            del layer1[idx1]
            layer0[idx1] += layer0[idx1+1]
            del layer0[idx1+1]
        else:
            layer0_idx1 = layer0[idx1]
            del layer0_idx1[bisect_left(layer0_idx1, x)]

    def bisect_left(self, x):  # x 以上の最小の値を返す  O(log(n))
        raise NotImplementedError
        layer1, layer0 = self.layer1, self.layer0
        idx1 = bisect_left(layer1, x)
        res = layer1[idx1]
        if res == x:
            return res
        layer0_idx1 = layer0[idx1]
        if layer0_idx1:
            idx0 = bisect_left(layer0_idx1, x)
            if idx0 == len(layer0_idx1):
                return res
            else:
                return layer0_idx1[idx0]
        else:
            return res

    def search_higher(self, x):  # x を超える最小の値を返す  O(log(n))
        layer2, layer1, layer0 = self.layer2, self.layer1, self.layer0
        idx2 = bisect_right(layer2, x)
        l1 = layer1[idx2]
        idx1 = bisect_right(l1, x)
        la0 = layer0[idx2]
        l0 = la0[idx1]
        idx0 = bisect_right(l0, x)
        if idx0==len(l0):
            if idx1==len(l1):
                return layer2[idx2]
            return l1[idx1]
        return l0[idx0]

    def search_lower(self, x):  # x 未満の最大の値を返す  O(log(n))
        layer2, layer1, layer0 = self.layer2, self.layer1, self.layer0
        idx2 = bisect_left(layer2, x)
        l1 = layer1[idx2]
        idx1 = bisect_left(l1, x)
        la0 = layer0[idx2]
        l0 = la0[idx1]
        idx0 = bisect_left(l0, x)
        if idx0==0:
            if idx1==0:
                return layer2[idx2-1]
            return l1[idx1-1]
        return l0[idx0-1]

    def pop(self, idx):
        # 小さい方から idx 番目の要素を削除してその要素を返す（0-indexed）
        # O(sqrt(n))
        # for を回すので重め  使うなら square パラメータを大きめにするべき
        raise NotImplementedError
        layer1, layer0 = self.layer1, self.layer0
        s = -1
        for i, l0 in enumerate(layer0):
            s += len(l0) + 1
            if s >= idx:
                break
        if s==idx:
            layer0[i] += layer0[i+1]
            del layer0[i+1]
            return layer1.pop(i)
        else:
            return layer0[i].pop(idx-s)

    def print(self):
        print(self.layer2)
        print(self.layer1)
        print(self.layer0)


def main():
    # 参考: https://atcoder.jp/contests/abc140/submissions/7477790
    n = int(input())
    p = list(map(int, input().split()))
    idx = [0] * n
    for i in range(0, n):
        idx[i] = i
    idx.sort(key=lambda i: - p[i])
    ssl = CubeSkipList()
    ssl.add(-1)
    ssl.add(n)
    ans = 0
    for i in idx:
        nex = ssl.search_higher(i)
        nexnex = ssl.search_higher(nex)
        pre = ssl.search_lower(i)
        prepre = ssl.search_lower(pre)
        if prepre != float("inf"):
            ans += p[i] * (pre - prepre) * (nex - i)
        if nexnex != float("inf"):
            ans += p[i] * (i - pre) * (nexnex - nex)
        ssl.add(i)
    print(ans)


main()

Submission status: Accepted

Solution code:
import bisect


class SqrtSet:
    def __init__(self, block_limit=201):
        self.key = []
        self.child = [[]]
        self.block_limit = block_limit

    def search_lower(self, key):
        if key is None:
            return None
        ret = None
        i = bisect.bisect_left(self.key, key)
        if i != 0:
            ret = self.key[i - 1]
        block = self.child[i]
        i = bisect.bisect_left(block, key)
        if i != 0:
            ret = block[i - 1]
        return ret

    def search_higher(self, key):
        if key is None:
            return None
        ret = None
        i = bisect.bisect_right(self.key, key)
        if i != len(self.key):
            ret = self.key[i]
        block = self.child[i]
        i = bisect.bisect_right(block, key)
        if i != len(block):
            ret = block[i]
        return ret

    def insert(self, key):
        i = bisect.bisect(self.key, key)
        block = self.child[i]
        bisect.insort(block, key)
        if len(block) == self.block_limit:
            sep = self.block_limit // 2
            self.key.insert(i, block[sep])
            self.child.insert(i + 1, block[sep + 1:])
            self.child[i] = block[:sep]

    def dump(self):
        for b in self.child:
            print(len(b), end=" ")
        print("")


def main():
    n = int(input())
    p = list(map(int, input().split()))
    idx = [0] * n
    for i in range(0, n):
        idx[i] = i
    idx.sort(key=lambda i: - p[i])
    t = SqrtSet()
    t.insert(-1)
    t.insert(n)
    ans = 0
    for i in idx:
        nex = t.search_higher(i)
        nexnex = t.search_higher(nex)
        pre = t.search_lower(i)
        prepre = t.search_lower(pre)
        if prepre != None:
            ans += p[i] * (pre - prepre) * (nex - i)
        if nexnex != None:
            ans += p[i] * (i - pre) * (nexnex - nex)
        t.insert(i)
    print(ans)


main()