28 lines
688 B
C++
28 lines
688 B
C++
#include<stdio.h>
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#include<string.h>
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#include<iostream>
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#include<algorithm>
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using namespace std;
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int main()
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{
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int n, m, u, v, dp[110][110];
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while(~scanf("%d%d", &n,&m))
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{
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memset(dp, 0, sizeof(dp));
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for(int i=1; i<=m; i++)
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{
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scanf("%d%d", &v,&u);
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dp[u][v] = 1;
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}
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for(int k=1; k<=n; k++)
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for(int i=1; i<=n; i++)
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for(int j=1; j<=n; j++)
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dp[i][j] = max(dp[i][j], dp[i][k]&dp[k][j]);
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int flag = 0;
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for(int i=1; i<=n; i++)
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if(dp[i][i] == 1) flag = 1;
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printf(!flag ? "YES\n" : "NO\n");
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}
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return 0;
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}
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