Ratatoskr Gym - 101873H

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本文链接： https://blog.csdn.net/weixin_39132605/article/details/100522304

https://cn.vjudge.net/problem/Gym-101873H

理解：一条最长的路径的节点数量标志了，无论如何走都会经过其中一个标志节点；最优策略就是怎么标志这些节点了（不需要我们做）；

#include <iostream>
#include <cstring>
#include <queue>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <set>
#include <map>
#include <stack>
//#include <tr1/unordered_map>
#include <cmath>
//#include<bits/stdc++.h>
using namespace std;

#define sfi(i) scanf("%d",&i)
#define sfl(i) scanf("%I64d",&i)
#define sfs(i) scanf("%s",(i))
#define pri(i) printf("%d\n",i)
#define prl(i) printf("%I64d\n",i)
#define sff(i) scanf("%lf",&i)
#define ll long long
#define ull unsigned long long
#define uint unsigned int
#define mem(x,y) memset(x,y,sizeof(x))
#define INF 0x3f3f3f3f
#define inf 1e18
#define eps 1e-10
#define PI acos(-1.0)
#define lowbit(x) ((x)&(-x))
#define fl() printf("flag\n")
#define MOD(x) ((x%mod)+mod)%mod
#define endl '\n'
#define pb push_back
#define lson (rt<<1)
#define rson (rt<<1|1)
#define FAST_IO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)

template<typename T>inline void read(T &x)
{
    x=0;
    static int p;p=1;
    static char c;c=getchar();
    while(!isdigit(c)){if(c=='-')p=-1;c=getchar();}
    while(isdigit(c)) {x=(x<<1)+(x<<3)+(c-48);c=getchar();}
    x*=p;
}

//-----------------------------------------------

const int maxn=1e6+9;
const int mod=1e9+7;

int n;
vector<int >g[maxn];

int dfs(int u,int fa)
{
    int ans=1;
    for(auto x:g[u])
    {
        int v=x;
        if(v==fa) continue;
        ans=max(ans,dfs(v,u)+1);
    }
    return ans;
}

int main()
{
    //FAST_IO;
    //freopen("input.txt","r",stdin);

    cin>>n;
    int a,b,c;
    cin>>a>>b>>c;

    for(int i=0;i<n-1;i++)
    {
        int u,v;
        cin>>u>>v;
        g[u].pb(v);
        g[v].pb(u);
    }

    int ans=n;
    for(int i=1;i<=n;i++)
    {
        if(i==b||i==c) ans=min(ans,dfs(i,i)-1);
        else ans=min(ans,dfs(i,i));
    }

    cout<<ans<<endl;


    return 0;
}