Codeforces Gym 100342J Problem J. Triatrip 三元环

Posted 2020-07-30

题目链接：

http://codeforces.com/gym/100342

题意：

求三元环的个数

题解：

用bitset分别统计每个点的出度的边和入度的边。

枚举每一条边(a,b)，计算以b为出度的边的终点构成的点集和以a为入度的边的起点够成的点集的交集，更新答案。

代码：

#include<iostream>
#include<cstring>
#include<cstdio>
#include<bitset>
using namespace std;

const int maxn = 1555;
typedef __int64 LL;

bitset<maxn> in[maxn], out[maxn], And;
char str[maxn][maxn];

int main() {
    freopen("triatrip.in", "r", stdin);
    freopen("triatrip.out", "w", stdout);
    int n;
    scanf("%d", &n);
    for (int i = 0; i < n; i++) scanf("%s", str[i]);
    LL ans = 0;
    for (int i = 0; i < n; i++){
        for (int j = 0; j < n; j++) {
            if (str[i][j] == ‘+‘) {
                in[j][i] = true;
                out[i][j] = true;
            }
        }
    }
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            if (str[i][j] == ‘+‘) {
                And = (in[i]) & (out[j]);
                ans += And.count();
            }
        }
    }
    printf("%I64d\n", ans / 3);
    return 0;
}