uva 10160

一开始写的代码加上各种剪枝后还是超时， 然后看了一下状态压缩后过了，两个代码的具体思想是一样的，状态压缩后可以大大提升性能

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <map>
#include <set>
#include <vector>
#include <sstream>
#include <string>
#include <cstring>
#include <algorithm>
#include <iostream>
#define maxn 200010
#define INF 0x7fffffff
#define inf 10000000
#define ull unsigned long long
#define ll long long
using namespace std;

ll st[40], vis[40];
int n, m;

bool dfs(ll state, int ans, int aim, int k)
{
    if(ans == aim)
    {
        if(state == ((ll)1 << n)-1) return true;
        return false;
    }
    for(int i = k; i <= n; ++ i)
    {
        if((state|vis[i]) != ((ll)1 << n)-1) return false;
        if((state|st[i]) == state) continue;
        if(dfs(state|st[i], ans+1, aim, i+1)) return true;
    }
    return false;
}
void init()
{
    memset(vis, 0, sizeof(vis));
    for(int i = 0; i < n; ++ i)
        st[i+1] = ((ll)1 << i);
}
int main()
{
    while(scanf("%d%d", &n, &m) == 2 && n+m)
    {
        init();
        for(int i = 0; i < m ; ++ i)
        {
            int a, b;
            scanf("%d%d", &a, &b);
            st[a] |= ((ll)1 << (b-1));
            st[b] |= ((ll)1 << (a-1));
        }
        vis[n] = st[n];
        for(int i = n-1; i > 0; -- i)
            vis[i] = vis[i+1]|st[i];
        for(int i = 1; i <= n; ++i)
        {
            if(dfs((ll)0, 0, i, 1))
            {
                printf("%d\n", i);
                break;
            }
        }
//        puts("*****************");
    }
    return 0;
}

\*time limit*\

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <map>
#include <set>
#include <vector>
#include <sstream>
#include <string>
#include <cstring>
#include <algorithm>
#include <iostream>
#define maxn 200010
#define INF 0x7fffffff
#define inf 10000000
#define ull unsigned long long
#define ll long long
using namespace std;

vector<int> g[40];
int vis[40];
bool mm[40][40];
int n, m;
bool checkall()
{
    for(int i = 1; i <= n; ++ i)
        if(!vis[i]) return false;
    return true;
}
bool checkii(int now)
{
    for(int i = now+1; i <= n; ++ i)
        if(mm[i][now]) return false;
    return true;
}
bool checknow(int now)
{
    if(!vis[now]) return true;
    for(int i = 0; i < (int)g[now].size(); ++ i)
        if(!vis[g[now][i]]) return true;
    return false;
}
void ok(int now)
{
    vis[now]++;
    for(int i = 0; i < (int)g[now].size(); ++ i)
        vis[g[now][i]]++;
}
void Nok(int now)
{
    vis[now]--;
    for(int i = 0; i < (int)g[now].size(); ++ i)
        vis[g[now][i]]--;
}
bool dfs(int ans, int aim, int k)
{
    if(ans == aim)
    {
        if(checkall()) return true;
        return false;
    }
    for(int i = k; i <= n; ++ i)
    {
        if(checknow(i))
        {
            ok(i);
            if(dfs(ans+1, aim, i+1)) return true;
            Nok(i);
        }
        if(!vis[i] && checkii(i)) return false;
    }
    return false;
}
void init()
{
    memset(vis, 0, sizeof(vis));
    memset(mm, 0, sizeof(mm));
    for(int i = 1; i <= n; ++ i) g[i].clear();
}
int main()
{
    while(scanf("%d%d", &n, &m) == 2 && n+m)
    {
        init();
        for(int i = 0; i < m ; ++ i)
        {
            int a, b;
            scanf("%d%d", &a, &b);
            g[a].push_back(b);
            g[b].push_back(a);
            mm[a][b] = mm[b][a] = true;
        }
        for(int i = 1; i <= n; ++i)
        {
            if(dfs(0, i, 1))
            {
                printf("%d\n", i);
                break;
            }
        }
    }
    return 0;
}