数组划分

注意两点 1 小心数组不能被3整除 2.即使cnt没有更新 也要将其赋进lcnt数组

#include <bits/stdc++.h>

using namespace std;
const int N = 100010;
typedef long long LL;
LL a[N];
LL s[N];
LL n;
LL ans;
LL lcnt[N];
LL cnt;
int main(){
   cin >> n;
    if(n <= 2){
        cout << "0";
        return 0;
    }
    
    for(int i = 1 ; i <= n ; i++){
        cin >> a[i];
        s[i] = s[i - 1] + a[i];
    }
    
    LL l = s[n] / 3;
    LL r = s[n] / 3 * 2;
    for(int i = 1 ; i <= n - 2 ; i++){
        if(s[i] == l){
            cnt++;
        } 
    lcnt[i] = cnt;
    }
    
    for(int i = 2 ; i <= n - 1 ; i++){
        if(s[i] == r){
            ans += lcnt[i - 1];
        }
    }
    if(s[n] % 3 != 0)cout << "0";
    else cout << ans;
    return 0;
}