# POJ 1915 Knight moves bi-directional BFS primer

Source: Internet
Author: User

Description

Background

Mr Somurolov, fabulous Chess-gamer Indeed, asserts that no one else but him can move knights from one position to a So fast. Can you beat him?

the Problem

Your task is to write a program to calculate the minimum number of moves needed for a knight Another, so and have the chance to be faster than Somurolov.

for people not familiar with chess, the possible knight moves are shown in Figure 1.

Input

the input begins with the number N of scenarios on a single line by itself.

Next follow n scenarios. Each scenario consists of three lines containing integer numbers. The the "specifies" of the length l of a side of the chess board (4 &lt;= L &lt;= 300). The entire board has size L * L. The second and third line contain pair of integers {0, ..., l-1}*{0, ..., l-1} specifying the starting and ending position Of the knight on the board. The integers are separated by a single blank. You can assume this positions are valid positions on the chess board of that scenario.

Output

for each scenario of the ' input ' have to calculate the minimal amount of knight moves which-are to move From the starting point to the ending point. If starting point and ending point are equal,distance is zero. The distance must is written on a single line.

Sample Input

 1 2 3 4 5 6 7 8 9 10 3 8 0 0 7 0 100 0 0 30 50 10 1 1-1 1

Sample Output

 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26-27--28 29---30 31--32 33 34 35 36 37 38-39 40 41 42 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 5 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100-101 5 0 <strong>: Give a n*n board and give the minimum number of steps to the end of the x y coordinate of the starting point. </strong> bidirectional BFS Code: <pre class= "Brush:java;" > #include <iostream> #include <cstdio> #include <queue> #include <cstring> #define QQ 330 using namespace Std; int vis1[qq][qq];     //Mark path also count step int vis2[qq][qq]; int fx1[8]={2,2,-2,-2,1,1,-1,-1}; int fx2[8]={1,-1,1,-1,2,-2,2,-2}; struct Node { int x,y;} start,end;              //bi-directional BFS at both ends of the beginning       int Sx,sy,ex,ey; int m; BOOL Inside (int xx,int yy)           //Judging out of bounds {     if (xx>=0&&yy>=0&&xx<m&&yy<m) return= "true;=" "else=" "false;=" "}=" " Void= "" "dBFS () =" "{=" "int=" "i,tq,tw;=" "queue<node=" ">q,w;            //Two queues     start.x=sx;start.y=sy;     end.x=ex;end.y=ey;     q.push (start);     w.push (end);                   vis1[sx][sy]=0;        //The following steps are added starting from 0.     vis2[ex][ey]=0;     while (!q.empty () &&!w.empty ())     {         node Now,next;         tq=q.size ();     //To first judge the entire queue         while (tq--)         {           now=q.front ();           q.pop ();           if (Inside (NOW.X,NOW.Y) &&VIS2[NOW.X][NOW.Y] !=-1)    //Both ends of the beginning are through thisPoint..           {               printf ("%dn", Vis1[now.x][now.y]+vis2[now.x][now.y]);               return;           }            for (i=0;i<8;i++)           {               next.x=now.x+fx1[i];               next.y=now.y+fx2[i];               if (Inside (NEXT.X,NEXT.Y) &&VIS2[NEXT.X][NEXT.Y]!=-1)    //important, because the odd step when ...               {    & nbsp;             printf ("%dn", vis1[now.x][ NOW.Y]+1+VIS2[NEXT.X][NEXT.Y]);                    Return               }                if (Inside (NEXT.X,NEXT.Y) &&vis1[next.x][ Next.y]==-1)               {                   vis1[next.x][ next.y]=vis1[now.x][now.y]+1;                    Q.push (next);               }            }         }         tw=w.size ();         while (tw--)        /Ibid.         {           Now=w.front ();           w.pop ();           if (Inside (NOW.X,NOW.Y) &&VIS1[NOW.X][NOW.Y] !=-1)           {               printf ("%dn", Vis1[now.x][now.y]+vis2[now.x][now.y]);               return;           }            for (i=0;i<8;i++)           {              next.x=now.x+ Fx1[i];               next.y=now.y+fx2[i];               if (Inside (NEXT.X,NEXT.Y) &&vis1[next.x][next.y]!=-1)                {                   printf ("%dn", Vis2[now.x][now.y]+1+vis1[next.x][next.y]);                    Return               }                if (Inside (NEXT.X,NEXT.Y) &&vis2[next.x][ Next.y]==-1)                {                   vis2[next.x][next.y]=vis2[now.x][now.y]+1;                    W.push (next);               }            }               int main () {    int T;     scanf ("%d", &t);      while (t--)     {        scanf ("%d", &m);   &NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;SCANF ("%d%d%d%d", &sx,&sy,&ex,&ey);         memset (vis1,-1,sizeof (VIS1));     // Mark as not traversed          memset (Vis2,-1,sizeof (VIS2));         dbfs (); &NBSP;&NBSP;&NBSP;&NBSP}         return 0; } </m&&yy<m) ></cstring></queue></cstdio></iostream></pre><br The essence of the > bidirectional BFS is to start the search from the start point and find the shortest path when both ends are found. <br><br>

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