POJ 2762 Going from u to v or from v to u?

  1 #include <iostream>
  2 #include <cstdio>
  3 #include <cstring>
  4 #include <cmath>
  5 #include <algorithm>
  6 #include <climits>
  7 #include <vector>
  8 #include <queue>
  9 #include <cstdlib>
 10 #include <string>
 11 #include <set>
 12 #include <stack>
 13 #define LL long long
 14 #define pii pair<int,int>
 15 #define INF 0x3f3f3f3f
 16 using namespace std;
 17 const int maxn = 1005;
 18 struct arc{
 19     int to,next;
 20     arc(int x = 0,int y = -1){
 21         to = x;
 22         next = y;
 23     }
 24 };
 25 int dfn[maxn],low[maxn],belong[maxn],tot;
 26 int head[maxn],head2[maxn],in[maxn],scc,cnt;
 27 stack<int>stk;
 28 bool instack[maxn];
 29 arc e[500000];
 30 void add(int *hd,int u,int v){
 31     e[tot] = arc(v,hd[u]);
 32     hd[u] = tot++;
 33 }
 34 void tarjan(int u){
 35     dfn[u] = low[u] = ++cnt;
 36     stk.push(u);
 37     instack[u] = true;
 38     for(int i = head[u]; ~i; i = e[i].next){
 39         if(!dfn[e[i].to]){
 40             tarjan(e[i].to);
 41             if(low[e[i].to] < low[u]) low[u] = low[e[i].to];
 42         }else if(instack[e[i].to] && dfn[e[i].to] < low[u]) low[u] = dfn[e[i].to];
 43     }
 44     if(dfn[u] == low[u]){
 45         scc++;
 46         int v;
 47         do{
 48            v = stk.top();
 49            stk.pop();
 50            instack[v] = false;
 51            belong[v] = scc;
 52         }while(v != u);
 53     }
 54 }
 55 bool topsort(){
 56     queue<int>q;
 57     for(int i = 1; i <= scc; i++){
 58         if(in[i]) continue;
 59         q.push(i);
 60     }
 61     if(q.size() > 1) return false;
 62     while(!q.empty()){
 63         int u = q.front();
 64         q.pop();
 65         for(int i = head2[u]; ~i; i = e[i].next)
 66             if(--in[e[i].to] == 0) q.push(e[i].to);
 67         if(q.size() > 1) return false;
 68     }
 69     return true;
 70 }
 71 int main() {
 72     int t,u,v,n,m;
 73     scanf("%d",&t);
 74     while(t--){
 75         scanf("%d %d",&n,&m);
 76         for(int i = 0; i < maxn; i++){
 77             dfn[i] = low[i] = 0;
 78             instack[i] = false;
 79             in[i] = belong[i] = 0;
 80             head[i] = head2[i] = -1;
 81         }
 82         scc = cnt = tot = 0;
 83         while(!stk.empty()) stk.pop();
 84         for(int i = 0; i < m; i++){
 85             scanf("%d %d",&u,&v);
 86             add(head,u,v);
 87         }
 88         for(int i = 1; i <= n; i++)
 89             if(!dfn[i]) tarjan(i);
 90         if(scc == 1) {puts("Yes");continue;}
 91         for(int i = 1; i <= n; i++){
 92             for(int j = head[i]; ~j; j = e[j].next){
 93                 if(i == e[j].to || belong[i] == belong[e[j].to]) continue;
 94                 in[belong[e[j].to]]++;
 95                 add(head2,belong[i],belong[e[j].to]);
 96             }
 97         }
 98         if(topsort()) puts("Yes");
 99         else puts("No");
100     }
101     return 0;
102 }