题目链接：
题目链接：点击打开链接
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <iostream>
#include <map>
#include <set>
#include <math.h>
using namespace std;
#define inf 10000000
#define N 100050
#define ll __int64
ll n;
ll a[N], lsum[N], rsum[N];
ll lok[N], rok[N];
int main(){
ll i, j;
while(cin>>n) {
ll sum = 0;
bool siz = false;
for(i=1;i<=n;i++)cin>>a[i], sum += a[i], siz |= a[i];
if(!siz) {
cout<<(n-1)*(n-2)/2<<endl;
continue;
}
if(sum%3){puts("0");continue;}
sum/=3;
lsum[0] = 0;
for(i=1;i<=n;i++)lsum[i] = lsum[i-1]+a[i];
rsum[n+1] = 0;
for(i=n;i;i--)rsum[i] = rsum[i+1]+a[i];
memset(lok, 0, sizeof lok);
memset(rok, 0, sizeof rok);
for(i=1;i<=n;i++) {
if(lsum[i]==sum){
lok[i] = 1;
}
// if(i==n)lok[i]=0;
// lok[i] += lok[i-1];
} for(i=n;i;i--) {
if(rsum[i]==sum){
rok[i] = 1;
}
rok[i]+=rok[i+1];
}
ll ans = 0;
for(i = 1; i+2<=n; i++)
ans += lok[i] * rok[i+2];
// ll ans = (lok[n]*(lok[n]-1))/2;
cout<<ans<<endl;
}
return 0;
}
/*
7
-1 1 -1 1 -1 1 0
9
-5 -2 1 1 5 0 -4 4 0
100
3 0 -5 2 -3 -1 -1 0 -2 -5 -4 2 1 2 -2 -1 -1 -4 3 -1 -3 -1 5 0 -4 -4 -1 0 -2 -2 0 1 -1 -2 -1 -5 -4 -2 3 1 -3 0 -1 1 0 -1 2 0 -2 -1 -3 1 -2 2 3 2 -3 -5 2 2 -2 -2 1 2 -2 -1 3 0 -4 7 -2 2 1 4 -9 -1 -2 -1 0 -1 0 -2 -2 -1 1 1 -4 2 -3 -3 7 1 1 -3 -7 0 -2 0 5 -2 */