Robot Motion

Time Limit: 1000MS	Memory Limit: 10000K
Total Submissions: 12845	Accepted: 6234

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)

Source

題意：有一個N*M的區域，機器人從第一行的第幾列進入，該區域全部由'N' , 'S' , 'W' , 'E' ，走到某個區域的時候只能按照該區域指定的方向進行下一步，問你機器人能否走出該片區域，若不能，輸入開始繞圈的步數和圈的大小。

DFS還是BFS對於這題有影響嗎？

對，有，BFS 0ms！！DFS 16 ms！！！

搜尋我是渣渣，看兩份到嗎的資料，POJ。

BFS AC程式碼：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
struct node
{
    int x,y;
    node(int x=0,int y=0):x(x),y(y) {};
    //裝X的寫法
};
char a[15][15];
bool vis[15][15];
int step[15][15];
int n,m,s;
void BFS()
{
    queue<node>q;
    memset(vis,false,sizeof(vis));
    step[0][s-1]=0;
    q.push(node(0,s-1));
    int stepp=-1;
    while(!q.empty())
    {
        node now=q.front();
        q.pop();
        stepp++;
        if(now.x<0||now.x>=n||now.y<0||now.y>=m)
        {
            printf("%d step(s) to exit\n",stepp);
            return ;
        }
        if(vis[now.x][now.y])
        {
            printf("%d step(s) before a loop of %d step(s)\n",step[now.x][now.y],stepp-step[now.x][now.y]);
            return ;
        }
        vis[now.x][now.y]=true;
        step[now.x][now.y]=stepp;
        if(a[now.x][now.y]=='S')
        {
            q.push(node(now.x+1,now.y));
        }
        else if(a[now.x][now.y]=='N')
        {
            q.push(node(now.x-1,now.y));
        }
        else if(a[now.x][now.y]=='W')
        {
            q.push(node(now.x,now.y-1));
        }
        else if(a[now.x][now.y]=='E')
        {
            q.push(node(now.x,now.y+1));
        }
    }
}
int main()
{
    while(cin>>n>>m>>s,n,m,s)
    {
        for(int i=0; i<n; i++)
            cin>>a[i];
        BFS();
    }
    return 0;
}

/**
3 3 1
SWW
SSN
EEN
*/

DFS AC程式碼：15ms

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
char a[15][15];
bool vis[15][15];
int step[15][15];
int n,m,s,stepp;
void DFS(int x,int y,int t)
{
    stepp++;
    if(x<0||x>=n||y<0||y>=m)
    {
        printf("%d step(s) to exit\n",stepp);
        return ;
    }
    if(vis[x][y])
    {
        printf("%d step(s) before a loop of %d step(s)\n",step[x][y],stepp-step[x][y]);
        return ;
    }
    vis[x][y]=true;
    step[x][y]=stepp;
    int xx,yy;
    if(a[x][y]=='S')
    {
        //x+=1;
        xx=x+1;
        yy=y;
    }
    else if(a[x][y]=='N')
    {
        //x-=1
        xx=x-1;
        yy=y;
    }
    else if(a[x][y]=='W')
    {
        //y-=1;
        xx=x;
        yy=y-1;
    }
    else if(a[x][y]=='E')
    {
        //y+=1;
        xx=x;
        yy=y+1;
    }
    DFS(xx,yy,stepp+1);
}
int main()
{
    while(cin>>n>>m>>s,n,m,s)
    {
        for(int i=0; i<n; i++)
            cin>>a[i];
        stepp=-1;
        memset(vis,false,sizeof(vis));
        DFS(0,s-1,0);
    }
    return 0;
}