Partitions

• Time : $O(n)$
• Space : $O(1)$

int Solution::solve(int A, vector<int> &B) {
    long long sum = 0;
    for(auto b : B) sum += b;
    if(sum % 3) return 0;
    long long t = sum / 3;
    long long now = 0;
    int res = 0, dp = 0;
    for(int i = 0; i < A - 1; i++) {
        now += B[i];
        if(now == t * 2) res += dp;
        if(now == t) dp += 1;
    }
    return res;
}