HDU 2444：The Accomodation of Students（二分图判定+匹配）

http://acm.hdu.edu.cn/showproblem.php?pid=2444

题意：给出边，判断这个是否是一个二分图，并求最大匹配。

思路：先染色法求出是否是一个二分图，然后再匈牙利求出最大匹配。注意输出是“No"而不是”NO“！！！

 #include <cstdio>
 #include <cstring>
 #include <cmath>
 #include <cstdlib>
 #include <algorithm>
 #include <string>
 #include <iostream>
 #include <stack>
 #include <map>
 #include <queue>
 using namespace std;
 #define N 100010
 #define INF 0x3f3f3f3f
 struct node
 {
     int nxt, v;
 }edge[*];
 int head[], tot;
 int col[];
 bool vis[];
 int match[];

 void add(int u, int v)
 {
     edge[tot].v = v; edge[tot].nxt = head[u]; head[u] = tot++;
 }

 bool check(int u,int c)
 {
     col[u] = c;
     for(int i = head[u]; ~i; i = edge[i].nxt) {
         int v = edge[i].v;
         if(col[v] == c) return false;
         if(col[v] ==  && !check(v, -c)) return false;
     }
     return true;
 }

 bool dfs(int u)
 {
     for(int i = head[u]; ~i; i = edge[i].nxt) {
         int v = edge[i].v;
         if(!vis[v]) {
             vis[v] = ;
             if(match[v] == - || dfs(match[v])) {
                 match[v] = u;
                 return true;
             }
         }
     }
     return false;
 }

 int main()
 {
     int n, m;
     while(~scanf("%d%d", &n, &m)) {
         memset(match, -, sizeof(match));
         memset(head, -, sizeof(head));
         memset(col, , sizeof(col));
         tot = ;
         for(int i = ; i < m; i++) {
             int u, v;
             scanf("%d%d", &u, &v);
             add(u, v); add(v, u);
         }
         bool f = ;
         for(int i = ; i <= n; i++) {
             if(col[i] == ) {
                 if(!check(i, )) {
                     f = ;
                     puts("No");
                     break;
                 }
             }
         }
         if(!f) continue;
         int sum = ;
         for(int i = ; i <= n; i++) {
             memset(vis, , sizeof(vis));
             sum += dfs(i);
         }
         printf("%d\n", sum / );
     }
     return ;
 }