POJ 2594 Treasure Exploration
题意：有向无环图，求最少多少条路径能够覆盖整个图，点能够反复走
思路：和普通的最小路径覆盖不同的是，点能够反复走，那么事实上仅仅要在多一步。利用floyd求出传递闭包。然后依据这个新的图去做最小路径覆盖就可以
代码：
#include <cstdio>
#include <cstring>
#include <vector>
#include <algorithm>
using namespace std; const int N = 505; int n, m, g[N][N]; int left[N], vis[N]; bool dfs(int u) {
for (int i = 1; i <= n; i++) {
if (g[u][i] && !vis[i]) {
vis[i] = 1;
if (!left[i] || dfs(left[i])) {
left[i] = u;
return true;
}
}
}
return false;
} int hungary() {
int ans = 0;
memset(left, 0, sizeof(left));
for (int i = 1; i <= n; i++) {
memset(vis, 0, sizeof(vis));
if (dfs(i)) ans++;
}
return ans;
} int main() {
while (~scanf("%d%d", &n, &m) && n) {
int u, v;
memset(g, 0, sizeof(g));
while (m--) {
scanf("%d%d", &u, &v);
g[u][v] = 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", n - hungary());
}
return 0;
}