POJ1383 Labyrinth(树的直径：两次BFS)

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<queue>
using namespace std;
char map[1005][1005];
int dx[4] = { 0,0,1,-1 };
int dy[4] = { 1,-1,0,0 };
int d[1005][1005];
int col, row,fx,fy;

int bfs(int x,int y)
{
int ans = 0;
memset(d, -1, sizeof(d));
queue<int> que;
que.push(x);
que.push(y);
d[x][y] = 0;
while (!que.empty())
{
x = que.front(); que.pop();
y = que.front(); que.pop();
for (int i = 0; i < 4; i++)
{
int xx = x + dx[i];
int yy = y + dy[i];
if (d[xx][yy]==-1 && map[xx][yy]=='.' && xx >= 1 && xx <= row&&yy >= 1 && yy <= col)
{
que.push(xx);
que.push(yy);
d[xx][yy] = d[x][y] + 1;
if (ans < d[xx][yy])//记录最长的位置
{
ans = d[xx][yy];
fx = xx;
fy = yy;
}
}
}
}
return ans;
}

int main()
{
int t,i,j;
scanf("%d", &t);
while (t--)
{
scanf("%d%d", &col, &row);
for (i = 1; i <= row; i++)
scanf("%s", map[i] + 1);
for (i = 1; i <= row; i++)
for (j = 1; j <= col; j++)
if (map[i][j] == '.')
{
fx = i;
fy = j;
break;
}
int ans;
bfs(fx, fy);		//先以任意点搜索一次找到最长段的一端v
ans = bfs(fx, fy);	//再以v搜索找到最长段的另一端w
printf("Maximum rope length is %d.\n", ans);
}
return 0;
}