HDU 1518 经典DFS

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从大到小排序可以减少耗时，

从小到大会增加耗时;

注意回溯和剪枝

/* *HDU 1518 *fuqiang11 *DFS *2013/7/28*/#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>using namespace std;#define maxn 20+3int a[maxn];bool visit[maxn];int N,M,eva;int cmp(int x, int y){    return x > y;}int  DFS(int st,int sum, int num){    if(num == 4)    {        return 1;    }    for(int i = st; i <= M; i++)    {        if(!visit[i])        {            if(sum + a[i] == eva)            {                visit[i] = true;                if(DFS(1,0,num+1))  //搜索到一条边，继续搜（从起点开始搜索）                    return 1;                visit[i] = false;            }            else if(sum + a[i] < eva)            {                visit[i] = true;                if(DFS(i+1,sum+a[i],num))                    return 1;                visit[i] = false;            }        }    }    return 0;}int main(){#ifndef ONLINE_JUDGE    freopen("in","r",stdin);#endif    cin>>N;    while(N--)    {        cin>>M;        int sum = 0,MAX = 0;        for(int i = 1; i <= M; i++)        {            cin>>a[i];            sum += a[i];//            MAX = max(MAX,a[i]);        }        eva = sum / 4;        if(sum % 4 != 0 || M < 4)        {            puts("no");            continue;        }        memset(visit,false,sizeof(visit));        sort(a+1,a+M+1,cmp);        if(!DFS(1,0,0))            puts("no");        else            puts("yes");    }}