hiho_1050_树中的最长路

#pragma once
#pragma execution_character_set("utf-8")
// 本文件为utf-8 编码格式

#include<iostream>
#include<stdio.h>
#include<string.h>
using namespace std;
#define N 100005
int dp[N][2];
struct Edge{
	int to;
	int next;
};
Edge gEdges[2*N];
int gHead[N];
bool gVisited[N];
int gEdgeIndex;
void InsertEdge(int u, int v){
	int e = gEdgeIndex++;
	gEdges[e].to = v;
	gEdges[e].next = gHead[u];
	gHead[u] = e;

	e = gEdgeIndex++;
	gEdges[e].to = u;
	gEdges[e].next = gHead[v];
	gHead[v] = e;
}


pair<int, int> dfs(int root){
	if (dp[root][0] != -1 && dp[root][1] != -1){
		return pair<int, int>(dp[root][0], dp[root][1]);
	}
	gVisited[root] = true;
	int e = gHead[root];
	int max1 = 0, max2 = 0, max = 0, child_num = 0;
	for (; e != -1; e = gEdges[e].next){
		int v = gEdges[e].to;
		if (!gVisited[v]){
			pair<int, int> result = dfs(v);
			max = max > result.first ? max : result.first;
			if (max1 >= result.second){
				max2 = max2 > result.second ? max2 : result.second;
			}
			else{//求一组数中的第一大和第二大的数！！！ 注意次序
				/*
				//这样做，在处理第一个result的时候， max1和max2赋值为同一个...error
				max1 = result.second;
				max2 = max1;

				//这样做，考虑到第一个值的处理，但是 对max1和max2的更新次序错了。 仔细考虑...
				max1 = result.second;
				if(max1 != 0)
					max2 = max1;
				*/
				if (max1 != 0)
					max2 = max1;
				max1 = result.second;
			}
			child_num++;
		}
	}
	if (child_num == 0)
		dp[root][0] = dp[root][1] = 0;
	else if (child_num == 1){
		dp[root][0] = max;
		dp[root][1] = 1 + max1;
	}
	else{
		dp[root][1] = 1 + max1;
		dp[root][0] = max > (2 + max1 + max2) ? max : (2 + max1 + max2);		
	}
	
	return pair<int, int>(dp[root][0], dp[root][1]);
}

void Init(){
	memset(gVisited, false, sizeof(gVisited));
	memset(gHead, -1, sizeof(gHead));
	gEdgeIndex = 0;
	memset(gEdges, -1, sizeof(gEdges));
	memset(dp, -1, sizeof(dp));
}
int main(){
	int n, u, v;
	scanf("%d", &n);
	Init();
	for (int i = 1; i < n; i++){
		scanf("%d %d", &u, &v);
		InsertEdge(u, v);
	}
	pair<int, int> result = dfs(1);
	printf("%d\n", result.first>result.second ? result.first : result.second);
	return 0;
}