前面用搜索寫了這題，其實這題也可以用DLX寫的。

#include <iostream>
#include<stdio.h>
#include<cmath>
#include<string.h>
#include<algorithm>
#include<string>
#include<vector>
#include <cstdlib>
using namespace std;
const int maxn = 510;
const int maxnode= 510*510*10;

int U[maxnode],D[maxnode],R[maxnode],L[maxnode],row[maxnode],col[maxnode];
struct DLX
{
    int n,m,sz;

    int H[maxn],S[maxn];
    int ans;
    void init(int n,int m)
    {
        this->n=n;
        this->m=m;
        for(int i = 0; i <= m; i++)
        {
            S[i] = 0;
            U[i] = D[i] = i;
            L[i] = i-1;
            R[i] = i+1;
        }
        R[m] = 0;
        L[0] = m;
        sz = m;
        for(int i = 1; i <= n; i++)H[i] = -1;
    }
    void Link(int r,int c)
    {
        ++S[col[++sz]=c];
        row[sz] = r;
        U[sz] = U[c];
        D[U[c]] = sz;
        D[sz] = c;
        U[c] = sz;
        if(H[r] < 0)H[r] = L[sz] = R[sz] = sz;
        else
        {
            L[sz] = L[H[r]];
            R[L[H[r]]] = sz;
            R[sz] = H[r];
            L[H[r]] = sz;
        }
    }
    #define FOR(i,A,s) for(int i=A[s];i!=s;i=A[i])
    void remove(int c)
    {
        FOR(i,D,c)
        {
            L[R[i]]=L[i];
            R[L[i]]=R[i];
        }
    }
    void restore(int c)
    {
        FOR(i,U,c)
        {
            L[R[i]]=i;
            R[L[i]]=i;
        }

    }
    bool v[maxnode];
    int f()
    {
        int ret = 0;
        for(int c = R[0]; c != 0; c = R[c]) v[c] = true;
        for(int c = R[0]; c != 0; c = R[c])
            if(v[c])
            {
                ret++;
                v[c] = false;
                for(int i = D[c]; i != c; i = D[i])
                    for(int j = R[i]; j != i; j = R[j])
                        v[col[j]] = false;
            }
        return ret;
    }
    void dfs(int d)
    {
        if(d+f()>=ans)
            return ;
        if(R[0]==0)
        {
            ans=min(ans,d);
            return ;
        }
        int c=R[0];
        FOR(i,R,0)
        if(S[i]<S[c])
            c=i;

        FOR(i,D,c)
        {
            remove(i);
            FOR(j,R,i) remove(j);
            dfs(d+1);
            FOR(j,L,i) restore(j);
            restore(i);
        }

        return;
    }
    int solve()
    {
        dfs(0);
        return ans;
    }
};

const int mmax = 510;
struct node
{
    int en;
    int next;
}E[mmax*mmax];
int p[mmax];
int num;
void init()
{
    memset(p,-1,sizeof p);
    num=0;
};
void add(int st,int en)
{
    E[num].en=en;
    E[num].next=p[st];
    p[st]=num++;
}
int main()
{
    int n,m;
    while(~scanf("%d %d",&n,&m))
    {
        init();
        for(int i=0;i<m;i++)
        {
            int u,v;
            scanf("%d %d",&u,&v);
            add(u,v);
            add(v,u);
        }
        DLX X;
        X.init(n,m);
        for(int i=1;i<=n;i++)
        {
            for(int j=p[i];j+1;j=E[j].next)
                X.Link(i,j/2+1);
        }
        X.ans=min(30,n);
        printf("%d\n",X.solve());

    }
    return 0;
}