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

void Dfs(int x,int c)
{
    int y; vis[x]=instack[x]=true,color[x]=c;
    for (int i=head[x];i!=-1;i=nxt[i])
    {
        y=to[i]; if (y==father[x]) continue;
        if (!vis[y])
        {
            father[y]=x;
            Dfs(y,c^1);
            Odd[x]+=Odd[y];
            Even[x]+=Even[y];
        }
        else
        {
            if (instack[y])
            {
                if (color[x]==color[y]) Odd[x]++;
                else Even[x]++;
            }
            else
            {
                if (color[x]==color[y]) Odd[father[x]]--;
                else Even[father[x]]--;
            }
        }
    }
    if (Even[x] || Odd[x]>=2) flag2=true;
    if (Odd[x]) flag1=true;
    instack[x]=false;
}

参考BC别人的AC的代码 o(╯□╰)o

/*Author :usedrose  */
/*Created Time :2015/7/31 15:14:40*/
/*File Name :2.cpp*/
#pragma comment(linker, "/STACK:1024000000,1024000000") 
#include <cstdio>
#include <iostream>
#include <algorithm>
#include <sstream>
#include <cstdlib>
#include <cstring>
#include <climits>
#include <vector>
#include <string>
#include <ctime>
#include <cmath>
#include <deque>
#include <queue>
#include <stack>
#include <set>
#include <map>
#define INF 0x3f3f3f3f
#define eps 1e-8
#define pi acos(-1.0)
#define MAXN 200110
#define OK cout << "ok" << endl;
#define o(a) cout << #a << " = " << a << endl
#define o1(a,b) cout << #a << " = " << a << "  " << #b << " = " << b << endl
using namespace std;
typedef long long LL;

struct E{
    E(int u, int nx) {
        v = u, nxt = nx;
    }
    E(){}
    int v, nxt;
}edge[600010];

bool even, odd;
int n, m, head[MAXN], col[MAXN], pre[MAXN], tot, cnt;
vector<int> bel[MAXN];

void init()
{
    cnt = tot = even = odd = 0;
    memset(col, 0, sizeof(col));
    memset(head, -1, sizeof(head));
    for (int i = 0; i <= n; ++i) {
        bel[i].clear();
    }
}

void addedge(int u, int v)
{
    edge[tot] = E(v, head[u]);
    head[u] = tot++;
}

void dfs(int u, int fa)
{
    for (int i = head[u]; i != -1; i = edge[i].nxt)
    {
        if ((i ^ 1) == fa) continue;
        int v = edge[i].v;
        //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
        if (col[v] == col[u]) {
            odd = 1;
            ++cnt;
            int x = v;
            while (!even)
            {
                bel[x].push_back(cnt);
                if (bel[x].size() > 1)
                    even = true;
                x = pre[x];
                if (x == u || x == -1) break;
            }
        }
        //如果其他节点已经访问过， 并且颜色不想同， 说明存在偶环
        if (col[v] == 3 - col[u])
            even = 1;
        if (!col[v]) {
            col[v] = 3 - col[u];
            pre[v] = u;
            dfs(v, i);
        }
    }
}

int main()
{
    //freopen("data.in","r",stdin);
    //freopen("data.out","w",stdout);
    int T;
    scanf("%d", &T);
    while (T--) {
        scanf("%d%d", &n, &m);
        init();
        int u, v;
        while (m--) {
            scanf("%d%d", &u, &v);
            addedge(u, v);
            addedge(v, u);
        }
        for (int i = 1; i <= n; ++i)
            if (!col[i]) {
                col[i] = 1;
                pre[i] = -1;
                dfs(i, -1);
            }
        puts(odd ? "YES" : "NO");
        puts(even ? "YES" : "NO");
    }
    return 0;
}