判断一个图中任意两个点是否双向，就是判断一个图是不是强连通图，每一个点的Low值都是一样的，换言之low数组值都想等

#include<iostream>
#include<cstdio>
#include<list>
#include<algorithm>
#include<cstring>
#include<string>
#include<queue>
#include<stack>
#include<map>
#include<vector>
#include<cmath>
#include<memory.h>
#include<set>

#define ll long long

#define eps 1e-7

#define inf 0xfffffff
const ll INF = 1ll<<61;

using namespace std;

//vector<pair<int,int> > G;
//typedef pair<int,int > P;
//vector<pair<int,int> > ::iterator iter;
//
//map<ll,int >mp;
//map<ll,int >::iterator p;
//

typedef struct Node {
	int form;
	int to;
	int nex;
	int sign;
};

Node edge[100000 + 10];

int dfn[10000 + 5],low[10000 + 5],head[10000 + 5],Stack[10000 + 5];

bool vis[10000 + 5];

int tot;
int scc_num;
int stack_num;
int vis_time;

void clear() {
	memset(dfn,-1,sizeof(dfn));
	memset(head,-1,sizeof(head));
	memset(vis,false,sizeof(vis));
	memset(low,0,sizeof(low));
	memset(Stack,0,sizeof(Stack));
	tot = 0;
	scc_num = 0;
	stack_num = 0;
	vis_time = 0;
}

void addedge(int u,int v) {
	edge[tot].form = u;
	edge[tot].to = v;
	edge[tot].nex = head[u];
	head[u] = tot++;
}


void tarjan(int v) {
	dfn[v] = low[v] = ++vis_time;
	vis[v] = true;
	Stack[stack_num++] = v;
	for(int i=head[v];i!=-1;i=edge[i].nex) {
		int u = edge[i].to;
		if(dfn[u] == -1) {
			tarjan(u);
			low[v] = min(low[u],low[v]);
		}
		else if(vis[u])
			low[v] = min(low[v],dfn[u]);
	}
	int tmp;
	if(low[v] == dfn[v]) {
		scc_num++;
		do {
			tmp = Stack[--stack_num];
			vis[tmp] = false;
		}while(tmp != v);
	}
}

void cal(int n) {
	for(int i=1;i<=n;i++)
		if(dfn[i] == -1)
			tarjan(i);
}

int main() {
	int n,m;
	while(scanf("%d %d",&n,&m), n + m) {
		clear();
		while(m--) {
			int a,b;
			scanf("%d %d",&a,&b);
			addedge(a,b);
		}
		cal(n);
		if(scc_num == 1)
			puts("Yes");
		else
			puts("No");
	}
	return EXIT_SUCCESS;
}

HDU1269 迷宫城堡 强连通 Tarjan