HDU-2444 The Accomodation of Students

先用染色法判断是否存在奇数环，也就是方案是否可行。然后二分匹配。

#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <fstream>
#include <iostream>

#define rep(i, l, r) for(int i=l; i<=r; i++)
#define down(i, l, r) for(int i=l; i>=r; i--)
#define N 234
#define MAX 1<<30

using namespace std;
int read()
{
	int x=0, f=1; char ch=getchar();
	while (ch<'0' || ch>'9') { if (ch=='-') f=-1; ch=getchar(); }
	while (ch>='0' && ch<='9') { x=x*10+ch-'0'; ch=getchar(); }
	return x*f;
}

struct node{int y, n;} e[N*N]; int fir[N], en;
int n, m, k[N], c[N], ans;
bool f, b[N];

inline void Add(int x, int y)
{
	en++, e[en].y=y, e[en].n=fir[x], fir[x]=en;
	en++, e[en].y=x, e[en].n=fir[y], fir[y]=en;
}

void Search(int x)
{
	int o=fir[x], y=e[o].y;
	while (o)
	{
		if (c[y]==c[x]) f=true; 
		if (!c[y]) { c[y]=3-c[x]; Search(y); }
		o=e[o].n, y=e[o].y;
	}
}

bool Find(int x)
{
	int o=fir[x], y=e[o].y;
	while (o)
	{
		if (!b[y]) 
		{
			b[y]=1; if (!k[y] || Find(k[y])) { k[y]=x; return true; }
		}
		o=e[o].n, y=e[o].y;
	}
	return false;
}

int main()
{
	while (~scanf("%d%d", &n, &m))
	{
		f=false; ans=en=0; rep(i, 1, n) fir[i]=k[i]=c[i]=0;
		rep(i, 1, m) { int x=read(), y=read(); Add(x, y); }
		rep(i, 1, n) if (!c[i]) { c[i]=1; Search(i); }
		if (f) { printf("No\n"); continue; }
		rep(i, 1, n) if (c[i]==1) 
		{
			rep(j, 1, n) b[j]=0;
			if (Find(i)) ans++;
		}
		printf("%d\n", ans);
	}
	return 0;
}