UVA - 11183 Teen Girl Squad(最小树形图)

题目大意：给出一张有向图，0为根，求出最小树形图

解题思路：模版题

#include <cstdio>
#include <cstring>

#define N 1010
#define M 40010

struct Edge{
    int u, v, c;
}E[M];

int n, m;

void init (){
    scanf("%d%d", &n, &m);
    for (int i = 0; i < m; i++)  scanf("%d%d%d", &E[i].u, &E[i].v, &E[i].c);
}

#define INF 0x3f3f3f3f
int vis[N], pre[N], id[N], in[N];

int Directed_MST(int root, int n) {
    int ans = 0;
    int u, v, tmp;
    while (1) {
        for (int i = 0; i < n; i++)
            in[i] = INF;
        for (int i = 0; i < m; i++) {
            u = E[i].u;
            v = E[i].v;
            if (u != v && E[i].c < in[v]) {
                in[v] = E[i].c;
                pre[v] = u;
            }
        }

        for (int i = 0; i < n; i++) {
            if (i == root)
                continue;
            if (in[i] == INF)
                return -1;
        }

        memset(vis, -1, sizeof(vis));
        memset(id, -1, sizeof(id));
        int subnode = 0;
        in[root] = 0;

        for (int i = 0; i < n; i++) {
            ans += in[i];
            tmp = i;

            while (vis[tmp] != i && tmp != root && id[tmp] == -1) {
                vis[tmp] = i;
                tmp = pre[tmp];
            }

            if (id[tmp] == -1 && tmp != root) {
                u = pre[tmp];
                while (u != tmp) {
                    id[u] = subnode;
                    u = pre[u];
                }
                id[tmp] = subnode++;
            }
        }

        if (subnode == 0)
            break;

        for (int i = 0; i < n; i++)
            if (!(~id[i]))
                id[i] = subnode++;

        for (int i = 0; i < m; i++) {
            tmp = E[i].v;
            E[i].u = id[E[i].u];
            E[i].v = id[E[i].v];

            if (E[i].u != E[i].v) 
                E[i].c -= in[tmp]; 
        }

        root = id[root];
        n = subnode;
    }
    return ans;
}

int cas = 1;
void solve() {
    printf("Case #%d: ", cas++);

    int ans = Directed_MST(0,n);
        if (ans == -1)
            printf("Possums!\n");
        else
            printf("%d\n", ans);
}

int main() {
    int test;
    scanf("%d", &test);
    while (test--) {
        init();
        solve();
    }
    return 0;
}