SPOJ Problem 1436:Is it a tree

#include<cstdio>
#include<cstring>
int f[10005],x,y,i,j,n,m;
int getf(int s){return f[s]==s?s:f[s]=getf(f[s]);}
void exch(int x,int y){int t=y;y=x;x=t;}
int main(){
    scanf("%d%d",&n,&m);
    if (m!=n-1){printf("NO\n");return 0;}
    for (i=1;i<=n;i++)
        f[i]=i;
    for (i=1;i<=m;i++){
        scanf("%d%d",&x,&y);
        if (x>y)exch(x,y);
        if (getf(x)!=getf(y))f[y]=getf(x);
    }
    for (i=1;i<=n;i++)
        if(getf(i)!=1){printf("NO\n");return 0;}
    printf("YES\n");
}