1815A - Ian and Array Sorting - CodeForces Solution

#include <iostream>
#include <bits/stdc++.h>

#define ll long long int
using namespace std;

void mainSolve()
{

    ll n;
    cin >> n;

    vector<ll> v(n);
    for (int i = 0; i < n; i++) {
        cin >> v[i];
    }
    if(n % 2) {
        cout << "YES"   << endl;
        return;
    }

    for (int i = n - 2; i > 0;i--) {
        ll diff = v[i + 1] - v[i];
        v[i] += diff;
        v[i - 1] += diff;
    }

    if(v[0] > v[1]) {
        cout << "NO" << endl;
        return;
    }
    cout << "YES" << endl;

}
int main()
{

    ll t;
    cin >> t;

    while (t--)
    {
        mainSolve();
    }

    return 0;
}