hdu1269 移动城堡 联通分量 Tarjan

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思路：tarjan求强连通分量  判断ans==n?

卿学姐视频：http://www.bilibili.com/video/av7330663/

代码：

#include <bits/stdc++.h>
using namespace std;

const int INF = 1<<30;
const int maxn = 1e4+7;

int dfn[maxn],low[maxn],vis[maxn],tot,n,m,ans;
vector<int> E[maxn];
stack<int> S;

void tarjan(int x){
    low[x] = dfn[x] = ++tot;
    S.push(x);
    vis[x] = 1;

    for(int i=0; i<E[x].size(); i++){
        int v = E[x][i];

        if(!dfn[v]){
            tarjan(v);
            low[x] = min(low[x],low[v]);
        }else if(vis[v]){
            low[x] = min(low[x],dfn[v]);
        }
    }

    if(low[x] == dfn[x]){
        int cnt = 0;
        while(1){
            int now = S.top();
            S.pop();
            vis[x] = 0;
            cnt++;
            if(now == x) break;
        }

        if(cnt >= 1) ans = min(ans,cnt);
    }
}

int main(){
    
    while(scanf("%d%d",&n,&m)==2 && (n||m)){
        memset(dfn,0,sizeof(dfn));
        memset(low,0,sizeof(low));
        memset(vis,0,sizeof(vis));

        ans = INF;
        tot = 0;
        while(!S.empty()) S.pop();
        for(int i=0; i<maxn; i++) E[i].clear();

        for(int i=1; i<=m; i++){
            int u,v;
            scanf("%d%d",&u,&v);
            E[u].push_back(v);
        }
    
        for(int i=1; i<=n; i++){
            if(!dfn[i])
                tarjan(i);
        }
    
        cout << ((ans==n)?"Yes":"No") << endl;
    }

}