HDU 4738 Caocao's Bridges

求一下权值最小的桥。

第一坑：如果图不连通，不用派人去炸桥，直接输出0

第二坑：可能会有重边

第三坑：如果桥上没有士兵守着，那至少要派一个人去炸桥。

#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
#include<algorithm>
using namespace std;

const int maxn = 1000 + 10; //结点数量
const int Maxn = 2 * 1000 * 1000 + 10; //边的数量
int low[maxn];
int dfn[maxn];
int U[Maxn], V[Maxn], W[Maxn];//存初始边
int flag[Maxn];//判断第i条边是不是割边
int Father[maxn];
struct Edge
{
    int from, to, id, ans;//ans为1，表示这条边是割边
} edge[Maxn];
vector<int>G[maxn];//邻接表
int N, M;//N个结点，M条边
int tmpdfn;//时间戳
int tot;
int Start, End;



void init()
{
    for (int i = 0; i < maxn; i++) G[i].clear();
    for (int i = 0; i < maxn; i++) Father[i] = i;
    memset(low, 0, sizeof(low));
    memset(dfn, 0, sizeof(dfn));
    memset(flag, 0, sizeof(flag));
    low[1] = dfn[1] = 1;
    tmpdfn = 0;
    tot = 0;
}

void AddEdge(int u, int v)
{
    edge[tot].from = u;
    edge[tot].to = v;
    edge[tot].id = tot;
    edge[tot].ans = 0;
    G[u].push_back(tot);
    tot++;

    edge[tot].from = v;
    edge[tot].to = u;
    edge[tot].id = tot;
    edge[tot].ans = 0;
    G[v].push_back(tot);
    tot++;
}

int Tarjan(int u, int id)
{
    tmpdfn++;
    int lowu = dfn[u] = tmpdfn;
    for (int i = 0; i<G[u].size(); i++)
    {
        int B = G[u][i];
        if (!dfn[edge[B].to])
        {
            int lowv = Tarjan(edge[B].to, edge[B].id);
            lowu = min(lowu, lowv);
            if (lowv >= dfn[u])
            {
                if (lowv>dfn[u])
                    edge[B].ans = 1;
            }
        }
        else if (dfn[edge[B].to])
        {
            if (edge[B].id / 2 == id / 2) continue;
            lowu = min(lowu, dfn[edge[B].to]);
        }
    }

    low[u] = lowu;
    return lowu;
}

void Display_Cutting_edge()
{
    for (int i = 0; i<2 * M; i++)
    if (edge[i].ans)
        flag[edge[i].id / 2] = 1;
}

int Find(int x)
{
    if (x != Father[x]) Father[x] = Find(Father[x]);
    return Father[x];
}

int main()
{
    while (~scanf("%d%d", &N, &M))
    {
        if (!N&&!M) break;
        int JIHE = N;
        init();
        for (int i = 0; i < M; i++)
        {
            scanf("%d%d%d", &U[i], &V[i], &W[i]);
            int fu = Find(U[i]);
            int fv = Find(V[i]);
            if (fu != fv) Father[fu] = fv, JIHE--;
            AddEdge(U[i], V[i]);
        }
        if (JIHE != 1) 
        {
            printf("%d\n", 0);
            continue;
        }
        Start = 1;
        End = N;
        Tarjan(1, -1);
        Display_Cutting_edge();
        int Ans = 0x7FFFFFFF;
        for (int i = 0; i < M; i++)
        if (flag[i] && W[i] < Ans)
            Ans = W[i];
        if (Ans == 0x7FFFFFFF) printf("-1\n");
        else if (Ans == 0) printf("%d\n", 1);
        else printf("%d\n", Ans);
    }
    return 0;
}