[haoi2009]毛毛虫

[haoi2009]毛毛虫

#include<cstdio>
#include<iostream>
#include<cmath>
#include<algorithm>
#include<cstring>
#include<cstdlib>
#include<cctype>
#include<vector>
#include<stack>
#include<queue>
using namespace std;
#define enter puts("") 
#define space putchar(' ')
#define Mem(a, x) memset(a, x, sizeof(a))
#define rg register
typedef long long ll;
typedef double db;
const int INF = 0x3f3f3f3f;
const db eps = 1e-8;
const int maxn = 3e5 + 5;
inline ll read()
{
  ll ans = 0;
  char ch = getchar(), last = ' ';
  while(!isdigit(ch)) last = ch, ch = getchar();
  while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();
  if(last == '-') ans = -ans;
  return ans;
}
inline void write(ll x)
{
  if(x < 0) x = -x, putchar('-');
  if(x >= 10) write(x / 10);
  putchar(x % 10 + '0');
}

int n, m, w[maxn];

struct Edge
{
  int nxt, to;
}e[maxn << 1];
int head[maxn], ecnt = -1;
void addEdge(int x, int y)
{
  e[++ecnt] = (Edge){head[x], y};
  head[x] = ecnt;
}

int dp[maxn], ans = 0;
void dfs(int now, int _f)
{
  int Max1 = 0, Max2 = 0; 
  dp[now] = 1;
  for(int i = head[now], v; i != -1; i = e[i].nxt)
    {
      v = e[i].to;
      if(v == _f) continue;
      dfs(v, now);
      if(dp[v] > Max1) 
          Max2 = Max1, Max1 = dp[v];
      else 
           if(dp[v] > Max2) 
              Max2 = dp[v];
      dp[now] = max(dp[now], dp[v] + w[now] - 1);
      //如果now做为子结点，给其父亲的贡献只为其"最长链" 
    }
  ans = max(ans, Max1 + Max2 + w[now] - 1);
  //某个点，其最长链，次长链，加其所有子结点，再减2，再加1，1是它本身 
}

int main()
{
  Mem(head, -1);
  n = read(); m = read();
  for(int i = 1; i <= m; ++i)
    {
      int x = read(), y = read();
      w[x]++; w[y]++;
      addEdge(x, y); 
      addEdge(y, x);
    }
  dfs(1, 0);
  write(ans), enter;
  return 0;
}

#include<bits/stdc++.h>
using namespace std;
int hea[300010],nxt[600010],to[600010],a[300010],d[300010],mx,tot;
void fason(int a,int b)
{
    nxt[++tot]=hea[a];
    hea[a]=tot;
    to[tot]=b;
}
void dfs(int x,int fa)
{
    d[x]=a[x];
    for(int i=hea[x],y;(y=to[i]);i=nxt[i])
    {
        if(y==fa)continue;
        dfs(y,x);
        mx=max(d[x]+d[y],mx);
        d[x]=max(d[x],d[y]+a[x]-1);
		//a[x]指x有多少子结点，已包含y，现在去掉之 
    }
}
int main()
{
    int n,x,y;
    scanf("%d%*d",&n);
    for(int i=1;i<n;i++)
    {
        scanf("%d%d",&x,&y);
        fason(x,y);
        fason(y,x);
        a[x]++;
        a[y]++;
    }
    dfs(1,0);
  
    printf("%d
",mx);
    return 0;
}