SP1437 PT07Z - Longest path in a tree

jennie

树上dp求直径的模板

#include<iostream>
#include<cstdio>
#include<cstring>
#include<vector>
using namespace std;
int n;
vector<int> v[100001];
int x,y;
int f1[100001];
int f2[100001];
int ans;
void dfs(int x,int f){
	for(int i=0;i<v[x].size();++i){
		if(v[x][i]==f) continue;
		dfs(v[x][i],x);
		int tem=f1[v[x][i]]+1;
		if(tem>f1[x]){
			f2[x]=f1[x];
			f1[x]=tem;
		}else if(tem>f2[x]){
			f2[x]=tem;
		}
	}
	ans=max(ans,f2[x]+f1[x]);
}
int main(){
	scanf("%d",&n);
	for(int i=1;i<n;++i){
		scanf("%d%d",&x,&y);
		v[x].push_back(y);
		v[y].push_back(x);
	}
	dfs(1,0);
	cout<<ans;
	return 0;
	
}