hdu 5215

判断图中是否含有奇环或者偶环

二分图的染色思想 + 胡搞

记住。。如果一图是二分图，那么它一定没有奇环。如果一图没有奇环的话，那么它可以是二分图。

c++提交，不加黑科技要爆栈。。

大牛的DFS http://www.cnblogs.com/vb4896/p/4500177.html

 1 void Dfs(int x,int c)
 2 {
 3     int y; vis[x]=instack[x]=true,color[x]=c;
 4     for (int i=head[x];i!=-1;i=nxt[i])
 5     {
 6         y=to[i]; if (y==father[x]) continue;
 7         if (!vis[y])
 8         {
 9             father[y]=x;
10             Dfs(y,c^1);
11             Odd[x]+=Odd[y];
12             Even[x]+=Even[y];
13         }
14         else
15         {
16             if (instack[y])
17             {
18                 if (color[x]==color[y]) Odd[x]++;
19                 else Even[x]++;
20             }
21             else
22             {
23                 if (color[x]==color[y]) Odd[father[x]]--;
24                 else Even[father[x]]--;
25             }
26         }
27     }
28     if (Even[x] || Odd[x]>=2) flag2=true;
29     if (Odd[x]) flag1=true;
30     instack[x]=false;
31 }

View Code

参考BC别人的AC的代码 o(╯□╰)o

  1 /*Author :usedrose  */
  2 /*Created Time :2015/7/31 15:14:40*/
  3 /*File Name :2.cpp*/
  4 #pragma comment(linker, "/STACK:1024000000,1024000000") 
  5 #include <cstdio>
  6 #include <iostream>
  7 #include <algorithm>
  8 #include <sstream>
  9 #include <cstdlib>
 10 #include <cstring>
 11 #include <climits>
 12 #include <vector>
 13 #include <string>
 14 #include <ctime>
 15 #include <cmath>
 16 #include <deque>
 17 #include <queue>
 18 #include <stack>
 19 #include <set>
 20 #include <map>
 21 #define INF 0x3f3f3f3f
 22 #define eps 1e-8
 23 #define pi acos(-1.0)
 24 #define MAXN 200110
 25 #define OK cout << "ok" << endl;
 26 #define o(a) cout << #a << " = " << a << endl
 27 #define o1(a,b) cout << #a << " = " << a << "  " << #b << " = " << b << endl
 28 using namespace std;
 29 typedef long long LL;
 30 
 31 struct E{
 32     E(int u, int nx) {
 33         v = u, nxt = nx;
 34     }
 35     E(){}
 36     int v, nxt;
 37 }edge[600010];
 38 
 39 bool even, odd;
 40 int n, m, head[MAXN], col[MAXN], pre[MAXN], tot, cnt;
 41 vector<int> bel[MAXN];
 42 
 43 void init()
 44 {
 45     cnt = tot = even = odd = 0;
 46     memset(col, 0, sizeof(col));
 47     memset(head, -1, sizeof(head));
 48     for (int i = 0; i <= n; ++i) {
 49         bel[i].clear();
 50     }
 51 }
 52 
 53 void addedge(int u, int v)
 54 {
 55     edge[tot] = E(v, head[u]);
 56     head[u] = tot++;
 57 }
 58 
 59 void dfs(int u, int fa)
 60 {
 61     for (int i = head[u]; i != -1; i = edge[i].nxt)
 62     {
 63         if ((i ^ 1) == fa) continue;
 64         int v = edge[i].v;
 65         //如果与访问过的节点颜色相同，说明存在奇环，然后把所有在奇环上的点所在环的个数记录一下
 66         if (col[v] == col[u]) {
 67             odd = 1;
 68             ++cnt;
 69             int x = v;
 70             while (!even)
 71             {
 72                 bel[x].push_back(cnt);
 73                 if (bel[x].size() > 1)
 74                     even = true;
 75                 x = pre[x];
 76                 if (x == u || x == -1) break;
 77             }
 78         }
 79         //如果其他节点已经访问过， 并且颜色不想同， 说明存在偶环
 80         if (col[v] == 3 - col[u])
 81             even = 1;
 82         if (!col[v]) {
 83             col[v] = 3 - col[u];
 84             pre[v] = u;
 85             dfs(v, i);
 86         }
 87     }
 88 }
 89 
 90 int main()
 91 {
 92     //freopen("data.in","r",stdin);
 93     //freopen("data.out","w",stdout);
 94     int T;
 95     scanf("%d", &T);
 96     while (T--) {
 97         scanf("%d%d", &n, &m);
 98         init();
 99         int u, v;
100         while (m--) {
101             scanf("%d%d", &u, &v);
102             addedge(u, v);
103             addedge(v, u);
104         }
105         for (int i = 1; i <= n; ++i)
106             if (!col[i]) {
107                 col[i] = 1;
108                 pre[i] = -1;
109                 dfs(i, -1);
110             }
111         puts(odd ? "YES" : "NO");
112         puts(even ? "YES" : "NO");
113     }
114     return 0;
115 }