813C The Tag Game --- DFS

 追逐游戏啊，🚬 //沧桑脸

下回把dfs，bfs，dijkstra都写一遍~

//DFS和BFS区别

（1）dfs学了这篇：https://blog.csdn.net/mengxiang000000/article/details/72901402

但是。。。我没有理解遇到两人到某点距离一样，就更新答案这句，而且，不加这句也ac了，搞不明白<(＿　＿)>

#include<bits/stdc++.h>
#define mem(a) memset(a,0,sizeof(a))
#define ll long long
#define inf 0x3f3f3f3f
const int N=2e5+5;
const int M=1e3+10;
const ll lim=1e14+5;
using namespace std;
int ans,dis[2][N];
vector<int>ve[N];
void dfs(int now,int from,int judge,int sum)
{
   dis[judge][now]=sum;
   if(judge==1&&sum<dis[0][now])
   {
       ans=max(ans,dis[0][now]*2);//停
   }
   for(int i=0;i<ve[now].size();i++)
   {
        int now1=ve[now][i];
        if(now1==from) continue;
        dfs(now1,now,judge,sum+1);
   }
}
int main()
{
    int n,x,a,b;
    scanf("%d%d",&n,&x);
    for(int i=0;i<n-1;i++)
    {
        scanf("%d%d",&a,&b);
        ve[a].push_back(b);
        ve[b].push_back(a);
    }
    dfs(1,0,0,0);
    dfs(x,0,1,0);
    printf("%d\n",ans);
    return 0;
}