HDU 1272 小希的迷宫(并查集判断有无成环)

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int MAXN = 100005;
int pre[MAXN];
bool vis[MAXN];
int n,m,a,b,flag;

void init(){
  for(int i=0;i<=MAXN;i++){
    pre[i] = i;
    vis[i] = 0;
  }
}

int Find(int x){
  if(x != pre[x]){
    pre[x] = Find(pre[x]);
  }
  return pre[x];
}

void merge(int x,int y){
  int fx = Find(x);
  int fy = Find(y);
  if(fx != fy){
    pre[fy] = fx;
  }
  else flag = 1;            // 当出现有两个点的根节点相同就会有环出现
}

int main()
{
  while(~scanf("%d%d",&n,&m)){
    if(n == -1 && m == -1) break;
    if(n == 0 && m == 0){         // 特判
      printf("Yes\n");
      continue;
    }
    init();
    flag = 0;
    vis[n] = 1;
    vis[m] = 1;
    merge(n,m);
    while(~scanf("%d%d",&a,&b)){
      if(a == 0 && b == 0) break;
      vis[a] = 1;
      vis[b] = 1;
      merge(a,b);
    }
    if(flag){
      printf("No\n");
      continue;
    }
    else{
      int sum = 0;
      for(int i=0;i<=MAXN;i++){
        if(vis[i] && i == pre[i])sum++;
      }
      if(sum == 1)printf("Yes\n");
      else printf("No\n");
    }
  }
  return 0;
}