离散题目14

#include<stdio.h>

int main()
{
int n, m;
while(scanf("%d %d", &n, &m) != EOF)
{
int a[1005][1005];
int x, y;
while(m--)
{
scanf("%d %d", &x, &y);
a[x][y] = 1;
}
int flag = 1;
int i, j;
for(i = 1; i <= n; i++)
for(j = 1; j <= n; j++)
{
if(a[i][j] != a[j][i])
flag = 0;
}
if(flag)
printf("YES\n");
else
printf("NO\n");
}
return 0;
}