bzoj1770: [Usaco2009 Nov]lights 燈

#include<cstdio>
#include<cstring>
#include<bitset>
#include<iostream>
#include<algorithm>
using namespace std;
#define REP(i,s,t) for(int i=s;i<=t;i++)
#define dwn(i,s,t) for(int i=s;i>=t;i--)
#define clr(x,c) memset(x,c,sizeof(x))
int read(){
	int x=0;char c=getchar();
	while(!isdigit(c)) c=getchar();
	while(isdigit(c)) x=x*10+c-‘0‘,c=getchar();
	return x;
}
const int nmax=37;
const int inf=0x7f7f7f7f;
bitset<nmax>a[nmax];
int ans[nmax],res=inf,tot=0,N,M;
void Gauss(int n,int m){
	REP(i,1,n){
		int j;
		for(j=i;j<=n&&!a[j][i];j++);
		if(j>n) continue;
		if(j!=i) swap(a[i],a[j]);
		REP(j,i+1,n) if(a[j][i]) a[j]^=a[i];
	}
}
void dfs(int x){
	if(tot>=res) return ;
	if(!x){
		res=min(res,tot);return ;
	}
	if(a[x][x]){
		int t=a[x][N+1];
		REP(i,x+1,N) if(a[x][i]) t^=ans[i];
		ans[x]=t;
		if(t) tot++;
		dfs(x-1);
		if(t) tot--;
	}else{
		ans[x]=0;dfs(x-1);
		ans[x]=1;tot++;dfs(x-1);tot--;
	}
}
int main(){
	N=read(),M=read();int u,v;
	REP(i,1,N) a[i].reset(),a[i][i]=a[i][N+1]=1;
	REP(i,1,M) u=read(),v=read(),a[u][v]=a[v][u]=1;
	Gauss(N,N+1);
	dfs(N);
	printf("%d\n",res);
	return 0;
}