POJ 2243 Knight Moves（BFS）

                                                                                 Knight Moves

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 13192		Accepted: 7395

Description

A friend of you is doing research on the Traveling Knight Problem (TKP) where you are to find the shortest closed tour of knight moves that visits each square of a given set of n squares on a chessboard exactly once. He thinks that the most difficult part of the problem is determining the smallest number of knight moves between two given squares and that, once you have accomplished this, finding the tour would be easy.
Of course you know that it is vice versa. So you offer him to write a program that solves the "difficult" part.

Your job is to write a program that takes two squares a and b as input and then determines the number of knight moves on a shortest route from a to b.

Input

The input will contain one or more test cases. Each test case consists of one line containing two squares separated by one space. A square is a string consisting of a letter (a-h) representing the column and a digit (1-8) representing the row on the chessboard.

Output

For each test case, print one line saying "To get from xx to yy takes n knight moves.".

Sample Input

e2 e4
a1 b2
b2 c3
a1 h8
a1 h7
h8 a1
b1 c3
f6 f6

Sample Output

To get from e2 to e4 takes 2 knight moves.
To get from a1 to b2 takes 4 knight moves.
To get from b2 to c3 takes 2 knight moves.
To get from a1 to h8 takes 6 knight moves.
To get from a1 to h7 takes 5 knight moves.
To get from h8 to a1 takes 6 knight moves.
To get from b1 to c3 takes 1 knight moves.
To get from f6 to f6 takes 0 knight moves.

解题思路：          是一道比较基础的搜索问题，只需考虑马的跳跃规则，而不需要考虑蹩马腿的情况，有一点特殊的地方就在于，这道题并没有给出明确的图，所以需要自己构造一张图，当然其实只需要考虑是否被重复走过就可以了。

BFS代码：

#include <iostream>
#include <cstdio>
#include <queue>
#include <cstring>
using namespace std;

struct point
{
    int x,y;
};
//int sx,sy,ex,ey;//s->start y->end

point s,e;
int step[10][10];
string map[10];
int dx[8] = {1,1,-1,-1,2,2,-2,-2};
int dy[8] = {2,-2,2,-2,1,-1,1,-1};
int BFS(point s)
{
    queue<point>P;
    //memset(step,0,sizeof(step));
    int i;
    P.push( s );
    point hd;
    step[s.x][s.y] = 0;
    while(!P.empty())
    {
        hd = P.front();
        P.pop();
        if(hd.x==e.x && hd.y==e.y)
            return step[hd.x][hd.y];

        for(i=0;i<8;i++)
        {
            int x = hd.x + dx[i];
            int y = hd.y + dy[i];
            if(x>=1 && x<=8 && y>=1 && y<=8 && map[x][y]!='@')
            {
                step[x][y] = step[hd.x][hd.y] + 1;
                map[x][y] = '@';

                point t;
                t.x = x;
                t.y = y;
                P.push(t);
            }
        }
    }
}
int main()
{
    //freopen("test.txt","r",stdin);
    char s1[5],s2[5];
    int i;
    while(cin>>s1>>s2)//~scanf("%s %s",s1,s2)
    {
        s.x = s1[1] - '0';
        s.y = s1[0] - 'a' + 1;
        e.x = s2[1] - '0';
        e.y = s2[0] - 'a' + 1;
        for(i=1;i<=8;i++)
        map[i] = ".........";
        printf("To get from %s to %s takes %d knight moves.\n",s1,s2,BFS( s ));
    }
    //cout << "Hello world!" << endl;
    return 0;
}