[Bzoj1143][CTSC2008]祭祀river

题目链接

思路清晰的发现题意是求最小路径覆盖。

最少路径覆盖：是指在一个有向图中，找出最少的几条路径，用它们来覆盖全图。

先闭包处理一下，然后二分图跑一下。

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const unsigned int maxn = 100 + 10;
const double PI = acos(-1);
int g[maxn][maxn], L[maxn], vis[maxn];
int n;
int dfs(int x) {
    for (int i = 1; i <= n; i++) {
        if (g[x][i] && vis[i] == 0) {
            vis[i] = 1;
            if (L[i] == -1 || dfs(L[i])) {
                L[i] = x;
                return 1;
            }
        }
    }
    return 0;
}
int xyl() {
    int ans = 0;
    memset(L, -1, sizeof(L));
    for (int i = 1; i <= n; i++) {
        memset(vis, 0, sizeof(vis));
        if (dfs(i))
            ans++;
    }
    return ans;
}
int main() {
    int  m, x, y;
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= m; i++)
        scanf("%d%d", &x, &y), g[x][y] = 1;
    for (int k = 1; k <= n; k++)
        for (int i = 1; i <= n; i++)
            for (int j = 1; j <= n; j++)
                g[i][j] |= g[i][k] & g[k][j];
    printf("%d
", n - xyl());
}