hihocoder #1224 : 赛车 dfs

题目链接:

http://hihocoder.com/problemset/problem/1224

题意:

题解:

首先我们先dfs出最长链，然后我们再dfs出每一个点能够最长延展多少
然后最后答案就是最长链的长度+最长延展多少就好了

代码:

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define MS(a) memset(a,0,sizeof(a))
#define MP make_pair
#define PB push_back
const int INF = 0x3f3f3f3f;
const ll INFLL = 0x3f3f3f3f3f3f3f3fLL;
inline ll read(){
    ll x=0,f=1;char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
//////////////////////////////////////////////////////////////////////////
const int maxn = 1e5+10;

vector<int> G[maxn];
int d[maxn],vis[maxn],fa[maxn];

void dfs(int x,int deep){
    d[x] = deep;
    for(auto v : G[x]){
        if(vis[v]) continue;

        fa[v] = x;
        dfs(v,deep+1);
    }
}

int main(){
    int n=read();
    for(int i=1; i<n; i++){
        int u=read(),v=read();
        G[u].push_back(v);
    }
    dfs(1,0);

    int bf=1,ans1 = 0;
    for(int i=1; i<=n; i++){
        if(d[i] > ans1){
            ans1 = d[i];
            bf = i;
        }
    }

    while(bf != 1){
        vis[bf] = 1;
        bf = fa[bf];
    }
    vis[1] = 1;

    for(int i=1; i<=n; i++){
        if(vis[i])
            dfs(i,0);
    }

    int ans2 = 0;
    for(int i=1; i<=n; i++)
        if(!vis[i])
            ans2 = max(d[i],ans2);

    cout << ans1+ans2 << endl;

    return 0;
}