hdu1272 小希的迷宫 ——并查集无向图判环

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1272 题目大意： 　　给一个无向图，判断是不是有环，有就输出No，否则输出Yes 题目思路： 　　用并查集，开始还天真地以为要用拓扑排序，好吧……虽然那个也可以做，可是为什么不用简单的方法呢？

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cctype>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <vector>
#include <cmath>
#include <algorithm>
#define lson l, m, rt<<1
#define rson m+1, r, rt<<1|1
using namespace std;
typedef long long int LL;
const int MAXN =  0x7fffffff;
const int  MINN =  -0x7fffffff;
const double eps = 1e-9;
const int dir[8][2] = {{0,1},{1,0},{0,-1},{-1,0},{-1,1},
  {1,1},{1,-1},{-1,-1}};
const int MAX = 100000+10;
int parent[MAX];
bool flag;
void init() {
  for (int i = 1; i<=MAX; ++i) parent[i] = -1;
}
int find(int x) {
  int s;
  for (s = x; parent[s] != s; s = parent[s]) ;
  while (s != x) {
    int tmp = parent[x]; parent[x] = s; x = tmp;
  }
  return s;
}
void Union(int R1, int R2) {
  int r1 = find(R1), r2 = find(R2);
  if (r1 != r2) parent[r1] = r2;
  else flag = false;
}
int main(void){
#ifndef ONLINE_JUDGE
  freopen("hdu1272.in", "r", stdin);
#endif
  int a, b; bool re = true;
  map<int, bool> mymap;
  while (1) {
    init(); flag = true;
    mymap.clear();
    while (~scanf("%d%d", &a, &b)) {
      if (a == 0 && b == 0) break;
      if (!mymap[a]) {parent[a] = a;mymap[a] = true;}
      if (!mymap[b]) {parent[b] = b;mymap[b] = true;}
      if (a == -1 && b == -1) {re = false; break;}
      Union(a, b);
    }
    if (!re) break;
    int cnt = 0;
    for (int i = 1; i < MAX; ++i) {
      if (mymap[i] && parent[i] == i) {
        cnt++;
      }
    }
    if (cnt > 1) flag = false;
    if (flag) printf("Yes\n"); else printf("No\n");
  }

  return 0;
}

代码比较挫……也比较乱。。这题目就是输入的时候需要处理一下，别的没什么。