Codeforces Round #266 (Div. 2) C Number of Ways

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本文介绍了一个使用C++编写的程序，该程序能够读取一系列整数并计算这些整数是否能被等分为三组，每组总和相等。通过两次遍历数组，分别从前向后和从后向前累计求和，实现了高效查找。

#include<cstdio>
#include<cstring>
#include<cmath>
#include<climits>
#include<cctype>
#include<cstdlib>
#include<iostream>
#include<algorithm>
#include<queue>
#include<vector>
#include<map>
#include<set>
#include<string>
#include<stack>
#define ll __int64
#define MAX 1000000
#define INF INT_MAX
#define eps 1e-8

using namespace std;

ll a[MAX],b[MAX],c[MAX];

int cnt[MAX];

int main(){
	int n;
	while (scanf("%d",&n) != EOF){
		b[0] = c[n+1] = 0;
		ll sum = 0;
		memset(cnt,0,sizeof(cnt));
		for (int i = 1; i<=n; i++){
			scanf("%I64d",&a[i]);
			sum += a[i];
			b[i] = b[i-1] + a[i];
		}
		ll s = sum / 3;
		if (sum % 3 != 0){
			printf("0\n");
			continue;
		}
		for (int i = n; i>=1; i--)
			c[i] = c[i+1] + a[i];
		for (int i = n; i>=1; i--)
			if (c[i] == s)
				cnt[i] = cnt[i+1] + 1;
			else 
				cnt[i] = cnt[i+1];
		ll ans = 0;
		for (int i = 1; i<n-1; i++)
			if (b[i] == s)
				ans += cnt[i+2];
		printf("%I64d\n",ans);
	}
	return 0;
}