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?

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstdlib>
 4 #include <cstring>
 5 #include <cctype>
 6 #include <stack>
 7 #include <queue>
 8 #include <deque>
 9 #include <map>
10 #include <set>
11 #include <vector>
12 #include <cmath>
13 #include <algorithm>
14 #define lson l, m, rt<<1
15 #define rson m+1, r, rt<<1|1
16 using namespace std;
17 typedef long long int LL;
18 const int MAXN =  0x7fffffff;
19 const int  MINN =  -0x7fffffff;
20 const double eps = 1e-9;
21 const int dir[8][2] = {{0,1},{1,0},{0,-1},{-1,0},{-1,1},
22   {1,1},{1,-1},{-1,-1}};
23 const int MAX = 100000+10;
24 int parent[MAX];
25 bool flag;
26 void init() {
27   for (int i = 1; i<=MAX; ++i) parent[i] = -1;
28 }
29 int find(int x) {
30   int s;
31   for (s = x; parent[s] != s; s = parent[s]) ;
32   while (s != x) {
33     int tmp = parent[x]; parent[x] = s; x = tmp;
34   }
35   return s;
36 }
37 void Union(int R1, int R2) {
38   int r1 = find(R1), r2 = find(R2);
39   if (r1 != r2) parent[r1] = r2;
40   else flag = false;
41 }
42 int main(void){
43 #ifndef ONLINE_JUDGE
44   freopen("hdu1272.in", "r", stdin);
45 #endif
46   int a, b; bool re = true;
47   map<int, bool> mymap;
48   while (1) {
49     init(); flag = true;
50     mymap.clear();
51     while (~scanf("%d%d", &a, &b)) {
52       if (a == 0 && b == 0) break;
53       if (!mymap[a]) {parent[a] = a;mymap[a] = true;}
54       if (!mymap[b]) {parent[b] = b;mymap[b] = true;}
55       if (a == -1 && b == -1) {re = false; break;}
56       Union(a, b);
57     }
58     if (!re) break;
59     int cnt = 0;
60     for (int i = 1; i < MAX; ++i) {
61       if (mymap[i] && parent[i] == i) {
62         cnt++;
63       }
64     }
65     if (cnt > 1) flag = false;
66     if (flag) printf("Yes\n"); else printf("No\n");
67   }
68 
69   return 0;
70 }

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。

posted on 2013-05-12 23:44  aries__liu  阅读(432)  评论(0编辑  收藏  举报