HDU_1175_dfs

http://acm.hdu.edu.cn/showproblem.php?pid=1175

dfs(x,y,i,num)，xy表示位置，i表示方向，num表示转向次数，num=2时候的剪枝很重要。

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int a[1005][1005],vis[1005][1005],n,m,endx,endy,dis[4][2] = {{-1,0},{0,-1},{1,0},{0,1}},flag;

void dfs(int x,int y,int dir,int num)
{
    if(flag)    return;
    if(x == endx && y == endy)
    {
        flag = 1;
        return;
    }
    if(num == 2)
    {
        if(dir == 0||dir == 2)
        {
            if(y != endy)   return;
        }
        else
        {
            if(x != endx)   return;
        }
    }
    int xx,yy;
    for(int i = 0;i < 4;i++)
    {
        xx = x+dis[i][0];
        yy = y+dis[i][1];
        if(xx < 1 || xx > n || yy < 1 || yy > m || vis[xx][yy])  continue;
        if(xx == endx && yy == endy || a[xx][yy] == 0)
        {
            vis[xx][yy] = 1;
            if(i == dir)    dfs(xx,yy,dir,num);
            else if(num < 2)dfs(xx,yy,i,num+1);
            vis[xx][yy] = 0;
        }
    }

}
int main()
{
    while(cin >> n >> m && n &&m)
    {
        flag = 0;
        for(int i = 1;i <= n;i++)
        {
            for(int j = 1;j <= m;j++)   cin >> a[i][j];
        }
        int num;
        cin >> num;
        while(num--)
        {
            int beginx,beginy;
            cin >> beginx >> beginy >> endx >> endy;
            if(beginx == endx && beginy == endy || a[beginx][beginy] != a[endx][endy] || a[beginx][beginy] == 0)
            {
                cout << "NO" << endl;
                continue;
            }
            flag = 0;
            memset(vis,0,sizeof(vis));
            for(int i = 0;i < 4;i++)
            {
                if(!flag)   dfs(beginx,beginy,i,0);
            }
            if(flag)    cout << "YES" << endl;
            else    cout << "NO" << endl;
        }
    }
    return 0;
}