URAL - 1416 Confidential(次小生成树)

题目大意：要求你求出最小生成树和次小生成树

解题思路：比较裸的题

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
using namespace std;
const int MAXNODE = 510;
const int MAXEDGE = 1000010;
const int INF = 0x3f3f3f3f;

struct Edge {
    int u, v, d;
}E[MAXEDGE];

vector<Edge> G[MAXNODE];
int n, m;
bool visNode[MAXNODE], visEdge[MAXEDGE];
int f[MAXNODE], maxcost[MAXNODE][MAXNODE];

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

bool cmp(const Edge &a, const Edge &b) {
    return a.d < b.d;
}

void dfs(int s, int u, int Max, int fa) {
    maxcost[s][u] = max(maxcost[s][u], Max);
    for (int i = 0; i < G[u].size(); i++) {
        int v = G[u][i].v;
        if (v == fa) continue;
        int tmp = max(Max, G[u][i].d);
        dfs(s, v, tmp, u);
    }
}

int find(int x) {
    return x == f[x] ? x : f[x] = find(f[x]);
}

void solve() {
    for (int i = 1; i <= n; i++) {
        f[i] = i; G[i].clear();
        visNode[i] = false;
    }

    memset(visEdge, 0, sizeof(visEdge));
    sort(E, E + m, cmp);

    int cnt = 0, ans = 0;
    for (int i = 0; i < m; i++) {
        int tx = find(E[i].u);
        int ty = find(E[i].v);
        if (tx != ty) {
            ans += E[i].d;
            visEdge[i] = true;
            visNode[E[i].u] = visNode[E[i].v] = true;
            f[tx] = ty;
            G[E[i].u].push_back(E[i]);
            swap(E[i].u, E[i].v);
            G[E[i].u].push_back(E[i]);
            cnt++;
        }
    }

    for (int i = 1; i <= n; i++) 
        if (!visNode[i]) {
            printf("Cost: %d\n", -1);
            printf("Cost: %d\n", -1);
            return ;
        }

    if (cnt == m) {
        printf("Cost: %d\n", ans);
        printf("Cost: %d\n", -1);
        return ;
    }

    memset(maxcost, 0, sizeof(maxcost));
    for (int i = 1; i <= n; i++)
        dfs(i, i, 0, -1);

    int Min = INF;
    for (int i = 0; i < m; i++)
        if (!visEdge[i]) {
            Min = min(Min, E[i].d - maxcost[E[i].u][E[i].v]);
        } 
    printf("Cost: %d\n", ans);
    printf("Cost: %d\n", ans + Min);
}

int main() {
    while (scanf("%d%d", &n, &m) != EOF) {
        init();
        solve();
    }
    return 0;
}