HUD 2444 The Accomodation of Students （二分图染色+最大匹配）

#include<iostream>
#include<cstdio>
#include<cstring>
#define maxn 2010
using namespace std;
int n,m,num,head[maxn],f[maxn],match[maxn],color[maxn];
struct node
{
    int u,v,pre;
}e[maxn*maxn];
void Add(int from,int to)
{
    num++;
    e[num].u=from;
    e[num].v=to;
    e[num].pre=head[from];
    head[from]=num;
}
bool Color(int s)
{
    for(int i=head[s];i;i=e[i].pre)
      if(!color[e[i].v])
        {
          color[e[i].v]=-1*color[s];
          if(!Color(e[i].v))return 0;
        }
      else if(color[e[i].v]==color[s])return 0;
    return 1;
}
int Dfs(int s)
{
    for(int i=head[s];i;i=e[i].pre)
      if(f[e[i].v]==0)
        {
          f[e[i].v]=1;
          if(match[e[i].v]==0||Dfs(match[e[i].v]))//XXXXX又是这句错了 下次再写错就XXXX 
            {
              match[e[i].v]=s;
              return 1;
            }
        }
    return 0;
}
int main()
{
    while(~scanf("%d%d",&n,&m))
      {
          num=0;
          memset(f,0,sizeof(f));
          memset(head,0,sizeof(head));
          memset(color,0,sizeof(color));
          memset(match,0,sizeof(match));
        int u,v;
        for(int i=1;i<=m;i++)
          {
              scanf("%d%d",&u,&v);
              Add(u,v);Add(v,u);
          }
        if(n==1)
            {
                printf("No
");
                continue;
          }
        color[1]=1;
        if(!Color(1))
          {
              printf("No
");
              continue;
          }
        int ans=0;
        for(int i=1;i<=n;i++)
          {
              memset(f,0,sizeof(f));
              ans+=Dfs(i);
          }
        printf("%d
",ans/2);
      }
    return 0;
}