59 lines
1.3 KiB
C++
59 lines
1.3 KiB
C++
#include<iostream>
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#include<cstdio>
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#include<cstring>
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#include<algorithm>
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#include<queue>
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#include<vector>
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using namespace std;
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bool map[111][111];
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vector<int>vet[111];
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int from[111];
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int n,m;
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void floyd()
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{
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for(int k=0;k<n;k++)
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for(int i=0;i<n;i++)
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for(int j=0;j<n;j++)
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map[i][j]=(map[i][j]||(map[i][k]&&map[k][j]));
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}
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bool Solve()
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{
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queue<int>Q;
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int cnt=0;
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for(int i=0;i<n;i++){
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if(from[i]==0){ Q.push(i);cnt++; }
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}
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while(!Q.empty()){
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int u=Q.front();
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Q.pop();
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for(int i=0;i<vet[u].size();i++){
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int v=vet[u][i];
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if(from[v]>0){
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from[v]--;
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if(from[v]==0){ Q.push(v);cnt++; }
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}
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}
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}
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if(cnt==n)return true;
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return false;
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}
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int main(){
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int u,v;
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while(scanf("%d%d",&n,&m),n){
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memset(map,false,sizeof(map));
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memset(from,0,sizeof(from));
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for(int i=0;i<n;i++)vet[i].clear();
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while(m--){
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scanf("%d%d",&u,&v);
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map[u][v]=true;
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}
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for(int i=0;i<n;i++)
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for(int j=0;j<n;j++)if(i!=j){
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if(map[i][j]){ from[j]++;vet[i].push_back(j); }
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}
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bool flag=Solve();
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flag?puts("YES"):puts("NO");
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}
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return 0;
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}
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