树的高度：

题目：

 PS G:\vscodetest> .\ab.exe
5 5
1 2
1 4
1 5
2 3
3

#include <stdio.h>
#include <vector>
#include <queue>
using namespace std;
int main() {
    int n,m;
    scanf("%d %d",&n,&m);
    vector<vector<int>> tree(n + 1);
    for (int i = 0; i < n - 1; i++)
    {
        int u,v;
        scanf("%d %d",&u,&v);
        tree[u].push_back(v);
        tree[v].push_back(u);
    }
    
    queue<int> toVisit; //用来作BFS的队列
    toVisit.push(m);
    vector<int> distance(n + 1);
    //用来判断某个结点是否访问过
    //若访问过的话还可以记录根节点到该结点的距离
    for (int i = 0; i < n; i++)
    {
        distance[i] = -1;
    }
    distance[m] = 0; //先设置根节点到根节点的距离是0
    int maxdist = 0; //用于计数
    while (toVisit.empty() == false)
    {
        int current = toVisit.front();
        toVisit.pop();
        for (int i = 0; i < tree[current].size(); i++)
        {
            int child = tree[current][i];
            if (distance[child] != -1)
            {
                continue;
            }
            toVisit.push(child);
            distance[child] = distance[current] + 1;
            maxdist = distance[child];
        }
    }
    printf("%d\n",maxdist);
    return 0;    
}