poj 3740 Easy Finding

#include<iostream>            //按列dfs暴搜    
using namespace std;
int m,n,table[18][305],visited[18],tag[305],num;
void dfs(int start)
{
    int i,j,over;
    if(tag[start]==1)
        dfs(start+1);  
    else
        for(i=0;i<m;++i)
        {
            if(visited[i]==1||table[i][start]==0)
                continue;
            over=0;
            for(j=0;!over&&j<n;++j)
                if(table[i][j]+tag[j]>1)
                    over=1;
            if(over==1)
                continue;
            for(j=start;j<n;++j) 
                if(table[i][j]==1)
                    tag[j]=1,num++;
            visited[i]=1;
            if(num==n)
                return;
            dfs(start+1);
            if(num==n)
                return;
            for(j=start;j<n;++j)
                if(table[i][j]==1)
                    tag[j]=0,num--;
            visited[i]=0;
        }
}
int main()
{
    while(scanf("%d%d",&m,&n)!=EOF)
    {
        for(int i=0;i<m;++i)
            for(int j=0;j<n;++j)
                scanf("%d",&table[i][j]);
        memset(tag,0,sizeof(tag));
        memset(visited,0,sizeof(visited));
        num=0;
        dfs(0);
        if(n&&num==n)        //保证n不为0
            printf("Yes, I found it\n");
        else
            printf("It is impossible\n");        
    }
    return 0;
}