CodeForces 21C Stripe 2 构造题

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

*/