codeforce Number of Ways(暴力)

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define N 500005
using namespace std;
typedef long long LL;
LL prefix[N], suffix[N], num[N];
LL cntSuf[N];
int main(){
    int n;
    scanf("%d", &n);
    for(int i=1; i<=n; ++i){
        scanf("%lld", &num[i]);
        prefix[i]=prefix[i-1]+num[i];//前缀和
    }    
    for(int i=n; i>=1; --i)
        suffix[i]=suffix[i+1]+num[i];//后缀和
    LL s=prefix[n]/3;
    if(prefix[n]%3!=0){
        printf("0\n");
        return 0;
    }
    LL ans=0;    
    for(int i=1; i<=n; ++i)        
        if(suffix[n-i+1]==s) cntSuf[n-i+1]=cntSuf[n-i+2]+1;
        else cntSuf[n-i+1]=cntSuf[n-i+2];
    for(int i=1; i<=n; ++i)
        if(prefix[i]==s) ans+=cntSuf[i+2];
    printf("%lld\n", ans);
    return 0;
}