看代码：
#include <set>
#include <map>
#include <cmath>
#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
typedef long long LL;
using namespace std;
//const int inf=INT_MAX;
const int maxn=100000+100;
//#include <bits/stdc++.h>
int a[maxn];
int p1[maxn];
int main ()
{
int n;
while(~scanf("%d",&n))
{
memset(p1,0,sizeof(p1));
int sum=0;
for(int i=1;i<=n;i++)
{
scanf("%d",&a[i]);
sum+=a[i];
}
if(sum%3)
printf("0\n");
else
{
LL ans=0;
int key=sum/3;
int sum1=0;
int sum2=0;
for(int i=n;i>=1;i--)
{
sum2+=a[i];
if(sum2==key)
p1[i-1]=p1[i]+1;
else
p1[i-1]=p1[i];
}
for(int i=1;i<=n;i++)
{
sum1+=a[i];
if(sum1==key)
{
ans+=p1[i+1];
}
}
printf("%lld\n",ans);
}
}
return 0;
}