POJ 1573 Robot Motion (DFS)

Robot Motion
Time Limit: 1000MS Memory Limit: 10000K
Total Submissions: 12736 Accepted: 6171
Description

A robot has been programmed to follow the instructions in its path. Instructions for the next direction the robot is to move are laid down in a grid. The possible instructions are

N north (up the page)
S south (down the page)
E east (to the right on the page)
W west (to the left on the page)

For example, suppose the robot starts on the north (top) side of Grid 1 and starts south (down). The path the robot follows is shown. The robot goes through 10 instructions in the grid before leaving the grid.

Compare what happens in Grid 2: the robot goes through 3 instructions only once, and then starts a loop through 8 instructions, and never exits.

You are to write a program that determines how long it takes a robot to get out of the grid or how the robot loops around.
Input

There will be one or more grids for robots to navigate. The data for each is in the following form. On the first line are three integers separated by blanks: the number of rows in the grid, the number of columns in the grid, and the number of the column in which the robot enters from the north. The possible entry columns are numbered starting with one at the left. Then come the rows of the direction instructions. Each grid will have at least one and at most 10 rows and columns of instructions. The lines of instructions contain only the characters N, S, E, or W with no blanks. The end of input is indicated by a row containing 0 0 0.
Output

For each grid in the input there is one line of output. Either the robot follows a certain number of instructions and exits the grid on any one the four sides or else the robot follows the instructions on a certain number of locations once, and then the instructions on some number of locations repeatedly. The sample input below corresponds to the two grids above and illustrates the two forms of output. The word “step” is always immediately followed by “(s)” whether or not the number before it is 1.
Sample Input

3 6 5
NEESWE
WWWESS
SNWWWW
4 5 1
SESWE
EESNW
NWEEN
EWSEN
0 0 0
Sample Output

10 step(s) to exit
3 step(s) before a loop of 8 step(s)

题意：一个地图，地图上面每个点都会有这个点能走的方向，然后给你一个起点，问你什么时候能走出这个地图或者造成死循环

思路：直接DFS就好了，关于标记数组可以记录到达该点的时间，如果说再次到达该点说明造成了死循环，这样，用当前时间减去第一次到达该点的时间，就可以得出死循环的时间，然后第一次到达该点的时间即为死循环的开始时间.

ac代码：

//
//  main.cpp
//  First Project
//
//  Created by AnICoo1 on 16/7/24.
//  Copyright © 2016年 AnICoo1. All rights reserved.
//

#include<stdio.h>
#include<string.h>
#include<queue>
#include<algorithm>
#include<iostream>
using namespace std;
char s[1010][1010];
int n,m,k,flag1,flag2,v[1010][1010];
int BG,len,ans;
int dir[4][2]={{1,0},{-1,0},{0,1},{0,-1}};
int fun(char x)
{
    if(x=='N')
        return 1;
    else if(x=='S')
        return 0;
    else if(x=='E')
        return 2;
    else if(x=='W')
        return 3;
    return -1;
}
int check(int x,int y)
{
    if(x<0||x>=n||y<0||y>=m)
        return 1;
    return 0;
}
void DFS(int x,int y,int cnt)
{
    if(flag1!=0||flag2!=0) return ;
    if(check(x,y)!=0)
    {
        ans=cnt;
        flag1=1;
        return ;
    }
    if(v[x][y]!=-1)
    {
        flag2=1;
        BG=v[x][y];
        len=cnt-BG;
    }
    v[x][y]=cnt;
    int ch=fun(s[x][y]);
    int nx=x+dir[ch][0];
    int ny=y+dir[ch][1];
    DFS(nx,ny,cnt+1);
}
int main()
{
    while(scanf("%d%d%d",&n,&m,&k)!=EOF,n||m||k)
    {
        for(int i=0;i<n;i++) scanf("%s",s[i]);
        flag1=0;flag2=0;memset(v, -1, sizeof(v));
        DFS(0,k-1,0);
        if(flag1!=0)
        {
            printf("%d step(s) to exit\n",ans);
        }
        else if(flag2!=0)
        {
            printf("%d step(s) before a loop of %d step(s)\n",BG,len);
        }
    }
    return 0;
}