poj 1383 Labyrinth（树的直径详解，简单明了！）

Labyrinth

Time Limit: 2000MS Memory Limit: 32768KTotal Submissions: 4031 Accepted: 1516

Description

The northern part of the Pyramid contains a very large and complicated labyrinth. The labyrinth is divided into square blocks, each of them either filled by rock, or free. There is also a little hook on the floor in the center of every free block. The ACM have found that two of the hooks must be connected by a rope that runs through the hooks in every block on the path between the connected ones. When the rope is fastened, a secret door opens. The problem is that we do not know which hooks to connect. That means also that the neccessary length of the rope is unknown. Your task is to determine the maximum length of the rope we could need for a given labyrinth.

Input

The input consists of T test cases. The number of them (T) is given on the first line of the input file. Each test case begins with a line containing two integers C and R (3 <= C,R <= 1000) indicating the number of columns and rows. Then exactly R lines follow, each containing C characters. These characters specify the labyrinth. Each of them is either a hash mark (#) or a period (.). Hash marks represent rocks, periods are free blocks. It is possible to walk between neighbouring blocks only, where neighbouring blocks are blocks sharing a common side. We cannot walk diagonally and we cannot step out of the labyrinth. 
The labyrinth is designed in such a way that there is exactly one path between any two free blocks. Consequently, if we find the proper hooks to connect, it is easy to find the right path connecting them.

Output

Your program must print exactly one line of output for each test case. The line must contain the sentence "Maximum rope length is X." where Xis the length of the longest path between any two free blocks, measured in blocks.

Sample Input

23 3####.####7 6########.#.####.#.####.#.#.##.....########

Sample Output

Maximum rope length is 0.Maximum rope length is 8.

Hint

Huge input, scanf is recommended. 
If you use recursion, maybe stack overflow. and now C++/c 's stack size is larger than G++/gcc

题意：给出一个图，图中 . 表示路，#表示墙不能走，求出任意图中能走的最长的路的长度！

思路：树的直径！

     代码：

         树的直径！   ：任意找点进行bfs，找到最端点的一个点 ，从一个端点找到另一端点！，这样就行了，找到了最长的路径！

         

            

#include<stdio.h>#include<string.h>#include<queue>#include<iostream>#include<algorithm>using namespace std;#define  N  1100#define  INF  0x3f3f3f3fchar map[N][N];int maxstep,visit[N][N],n,m;struct node{int x;int y;int step;}num[N],ans;int move[4][2]={1,0,0,-1,0,1,-1,0};bool judge(node t){if(t.x>=0&&t.x <n&&t.y >=0&&t.y <m&&!visit[t.x ][t.y]&&map[t.x][t.y]=='.')return 1;return 0;}void  bfs(node start){int i,j,k;node p,q;queue<node> Q;memset(visit,0,sizeof(visit));while(!Q.empty() )Q.pop();Q.push(start);visit[start.x ][start.y ]=1;while(!Q.empty() ){q=Q.front() ;Q.pop();if(q.step>maxstep){maxstep=q.step ;ans=q;}for(i=0;i<4;i++){p.x =q.x +move[i][0];p.y =q.y +move[i][1];p.step=q.step +1;if(judge(p)){visit[p.x ][p.y ]=1;Q.push(p); }}}}int main(){int T;scanf("%d",&T);while(T--){int i,j,k,mark=0;        maxstep=0;        node start;scanf("%d%d",&m,&n); for(i=0;i<n;i++)scanf("%s",map[i]);for(i=0;i<n;i++){  for(j=0;j<m;j++)  {  if(map[i][j]=='.')  {  start.x =i;  start.y=j;  start.step =0;  bfs(start);//任意找点进行bfs，找到最端点的一个点   mark=1;  break;}  }  if(mark)  break;    }    memset(visit,0,sizeof(visit));    maxstep=-1; ans.step =0;    bfs(ans);//从一个端点找到另一端点！     printf("Maximum rope length is %d.\n", maxstep);}return  0;}