P1649 [USACO07OCT]障礙路線Obstacle Course

題目描述

Consider an N x N (1 <= N <= 100) square field composed of 1

by 1 tiles. Some of these tiles are impassible by cows and are marked with an ‘x‘ in this 5 by 5 field that is challenging to navigate:

. . B x . 
. x x A . 
. . . x . 
. x . . . 
. . x . . 
 

Bessie finds herself in one such field at location A and wants to move to location B in order to lick the salt block there. Slow, lumbering creatures like cows do not like to turn and, of course, may only move parallel to the edges of the square field. For a given field, determine the minimum number of ninety degree turns in any path from A to B. The path may begin and end with Bessie facing in any direction. Bessie knows she can get to the salt lick.

N*N(1<=N<=100)方格中，’x’表示不能行走的格子，’.’表示可以行走的格子。卡門很胖，故而不好轉彎。現在要從A點走到B點，請問最少要轉90度彎幾次？

輸入輸出格式

第一行一個整數N,下面N行，每行N個字符，只出現字符：’.’，’x’，’A’，’B’，表示上面所說的矩陣格子，每個字符後有一個空格。

【數據規模】

2<=N<=100

一個整數：最少轉彎次數。如果不能到達，輸出-1。

輸入輸出樣例

3
. x A
. . .
B x .

2

說明

【註釋】

只可以上下左右四個方向行走，並且不能走出這些格子之外。開始和結束時的方向可以任意。

思路：搜索。

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
int map[101][101];
int dx[4]={1,-1,0,0};
int dy[4]={0,0,1,-1};
int n,sx,sy,tx,ty,ans=0x7f7f7f7f;
void dfs(int x,int y,int tot,int pre){
    if(tot>ans)    return ;
    if(x==tx&&y==ty&&tot<ans){
        ans=tot;
        return ;
    }
    for(int i=0;i<4;i++){
        int cx=x+dx[i];
        int cy=y+dy[i];
        if(cx>=1&&cx<=n&&cy>=1&&cy<=n&&!map[cx][cy]){
            map[cx][cy]=1;
            if(i!=pre&&pre!=4)    dfs(cx,cy,tot+1,i);
            else dfs(cx,cy,tot,i);
            map[cx][cy]=0;
        }
    }
}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++) {
            char x;cin>>x;
            if(x==‘x‘)    map[i][j]=1;
            else map[i][j]=0;
            if(x==‘A‘){ sx=i;sy=j; }
            else if(x==‘B‘){ tx=i;ty=j; }
        }
    }
    dfs(sx,sy,0,4);
    if(ans!=2139062143)    cout<<ans;
    else cout<<"-1";
}
/*
5
. . B x .
. x x A .
. . . x .
. x . . .
. . x . .
*/

洛谷 P1649 [USACO07OCT]障礙路線Obstacle Course