LightOJ - 1380 Teleport(最小树形图)

#include <cstdio>
#include <cstring>

const int MAXNODE = 1010;
const int MAXEDGE = 10010;
typedef int Type;
const Type INF = 0x3f3f3f3f;

struct Edge {
int u, v;
Type dis;
Edge() {}
Edge(int u, int v, Type dis): u(u), v(v), dis(dis) {}
};

struct Directed_MT{
int n, m;
Edge edges[MAXEDGE];
int vis[MAXNODE];
int pre[MAXNODE];
int id[MAXNODE];
Type in[MAXNODE];

void init(int n) {
this->n = n;
m = 0;
}

void AddEdge(int u, int v, Type dis) {
edges[m++] = Edge(u, v, dis);
}

Type DirMt(int root) {
Type ans = 0;
while (1) {
//初始化
for (int i = 0; i < n; i++) in[i] = INF;

for (int i = 0; i < m; i++) {
int u = edges[i].u;
int v = edges[i].v;
//找寻最小入边，删除自环
if (edges[i].dis < in[v] && u != v) {
in[v] = edges[i].dis;
pre[v] = u;
}
}

//如果没有最小入边，表示该点不连通，则最小树形图形成失败
for (int i = 0; i < n; i++) {
if (i == root) continue;
if (in[i] == INF) return -1;
}

int cnt = 0;//记录缩点
memset(id, -1, sizeof(id));
memset(vis, -1, sizeof(vis));
in[root] = 0;//树根不能有入边
for (int i = 0; i < n; i++) {
ans += in[i];
int v = i;
//找寻自环
while (vis[v] != i && id[v] == -1 && v != root) {
vis[v] = i;
v = pre[v];
}
//找到自环
if (v != root && id[v] == -1) {
for (int u = pre[v]; u != v; u = pre[u])
id[u] = cnt;
id[v] = cnt++;
}
}

//如果没有自环了，表示最小树形图形成成功了
if (cnt == 0) break;

//找到那些不是自环的，重新给那些点进行标记
for (int i = 0; i < n; i++)
if (id[i] == -1) id[i] = cnt++;

for (int i = 0; i < m; i++) {
int v = edges[i].v;
edges[i].v = id[edges[i].v];
edges[i].u = id[edges[i].u];
if (edges[i].u != edges[i].v)
edges[i].dis -= in[v];
}
//缩点完后，点的数量就边了
n = cnt;
root = id[root];
}
return ans;
}
}MT;

int cas = 1;
void init() {
int n, m, k;
scanf("%d%d%d", &n, &m, &k);
MT.init(n);

int u, v, dis;
for (int i = 0; i < m; i++) {
scanf("%d%d%d", &u, &v, &dis);
MT.AddEdge(u, v, dis);
}
int ans = MT.DirMt(k);
if (ans == -1) printf("Case %d: impossible\n", cas++);
else printf("Case %d: %d\n", cas++, ans);

}

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