Ian and Array Sorting (Codeforces 1815A)
Tags: Math, greedy

To thank Ian, Mary gifted an array $$$a$$$ of length $$$n$$$ to Ian. To make himself look smart, he wants to make the array in non-decreasing order by doing the following finitely many times: he chooses two adjacent elements $$$a_i$$$ and $$$a_{i+1}$$$ ($$$1\le i\le n-1$$$), and increases both of them by $$$1$$$ or decreases both of them by $$$1$$$. Note that, the elements of the array can become negative.As a smart person, you notice that, there are some arrays such that Ian cannot make it become non-decreasing order! Therefore, you decide to write a program to determine if it is possible to make the array in non-decreasing order.

Input: The first line contains a single integer $$$t$$$ ($$$1 \leq t \leq 10^4$$$) — the number of test cases. The description of test cases follows.The first line of each test case consists of a single integer $$$n$$$ ($$$2\le n\le 3\cdot10^5$$$) — the number of elements in the array.The second line of each test case contains $$$n$$$ integers $$$a_1,a_2,\ldots,a_n$$$ ($$$1\le a_i\le 10^9$$$) — the elements of the array $$$a$$$.It is guaranteed that the sum of $$$n$$$ over all test cases does not exceed $$$3\cdot10^5$$$. 

Output: For each test case, output "YES" if there exists a sequence of operations which make the array non-decreasing, else output "NO".You may print each letter in any case (for example, "YES", "Yes", "yes", "yEs" will all be recognized as positive answer).

Note: For the first test case, we can increase $$$a_2$$$ and $$$a_3$$$ both by $$$1$$$. The array is now $$$[1, 4, 3]$$$.Then we can decrease $$$a_1$$$ and $$$a_2$$$ both by $$$1$$$. The array is now $$$[0, 3, 3]$$$, which is sorted in non-decreasing order. So the answer is "YES".For the second test case, no matter how Ian perform the operations, $$$a_1$$$ will always be larger than $$$a_2$$$. So the answer is "NO" and Ian cannot pretend to be smart.For the third test case, the array is already in non-decreasing order, so Ian does not need to do anything.

Sample Inputs:
5
3
1 3 2
2
2 1
4
1 3 5 7
4
2 1 4 3
5
5 4 3 2 1

Sample Outputs:
YES
NO
YES
NO
YES

Solution (Python):
for _ in range(int(input())):
    num_cases = int(input())
    numbers = list(map(int, input().split()))
    if sum(numbers[0::2]) <= sum(numbers[1::2]) or num_cases % 2:
        print('YES')
    else:
        print('NO')

Solution (Python):
for i in range(int(input())):
    a = int(input())
    b = list(map(int, input().split()))
    if sum(b[::2]) <= sum(b[1::2]) or a % 2 == 1:
        print('YES')
    else:
        print('NO')

Solution (Python):
for _ in range(int(input())):
    t = int(input())
    l = list(map(int, input().split()))
    if sum(l[0::2]) <= sum(l[1::2]) or t % 2:
        print('YES')
    else:
        print('NO')