2019 CCPC-江西省赛（填坑中）

#include <iostream>
#include <map>
#include <vector>
#include <queue>
#include <string>
#include <set>
#include <algorithm>
#include <cstring>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <list>
#include <deque>
#include <queue>
#include <stack>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <iomanip>
#pragma GCC optimize(2)
#define ll long long
#define ull unsigned long long
using namespace std;
template <class T>
void rd(T &x){
    x = 0 ;T flag = 1 ;char ch = getchar();
    while(!isdigit(ch)) { if(ch==‘-‘) flag = -1 ; ch = getchar() ; }
    while(isdigit(ch)) { x = (x<<1) + (x<<3) + (ch^48) ; ch = getchar(); }
    x*= flag ;
}
const int INF=0x3f3f3f3f;
const int mxn = 1e5+7;
ll t,n,m,k,cnt,node_1,node_2,ans_1,ans_2,mn_son = 0 , mn_node = 0 , res , ans ;
int head[mxn<<2] , to[mxn<<2] , nx[mxn<<2] ,deg[mxn<<2] ;
int vis[mxn<<2] ;
void add(int u,int v){ to[cnt] = v , nx[cnt] = head[u] , head[u] = cnt++; }
void DFS_INIT(int x) /// 计算两颗子树的节点数量
{
    if(vis[x]) return ;
    res++; vis[x] = 1 ;
    for(int i=head[x];~i;i=nx[i]) if(!vis[to[i]]) DFS_INIT(to[i]); /// 
}
void DFS(int x,int par,int n)
{
    deg[x]=1; int mx_son = 0 ;
    for(int i=head[x];~i;i=nx[i]){
        int v = to[i] ;
        if(v==par) continue;
        DFS(v,x,n);
        deg[x] += deg[v] ;
        mx_son = max(mx_son , deg[v]);/// 子树的最大节点个数
    }
    mx_son = max(mx_son,n-deg[x]); /// 子树的最大节点个数
    if( mx_son < mn_son ){
        mn_son = mx_son , mn_node = x ;
    }
}
void DFS_END(int x,int par)
{
    deg[x] = 1 ;
    for(int i=head[x];~i;i=nx[i]){
        int v = to[i] ;
        if(v==par) continue;
        DFS_END(v,x) ;
        deg[x] += deg[v];
        ans+=(ll)(deg[v])*(ll)(n-deg[v]);
    }
}
int main()
{
    while(~scanf("%lld",&n)){
        cnt = 0 ;
        memset(deg,0,sizeof(deg));
        memset(head,-1,sizeof(head));
        for(int i=1,u,v;i<=n-2;i++){
            rd(u) , rd(v) ;
            add(u,v) , add(v,u) ;
        }
        memset(vis,0,sizeof(vis));
        int n1 = 0 , n2 = 0 ; res = 0 ;
        DFS_INIT(1) , node_1 = 1 , n1 = res , res = 0 ;
        for(int i=1;i<=n;i++){
            if(!vis[i]) {
                DFS_INIT(i) , n2 = res , node_2 = i , res = 0 ;
                break;
            }
        }
        mn_son = INF ; int e1 , e2 ;
        DFS(node_1,0,n1) , e1 = mn_node ,mn_son = INF ;
        DFS(node_2,0,n2) , e2 = mn_node ,mn_son = INF ;
        ///cout<<e1<<" -- "<<e2<<endl;
        add(e1,e2) , add(e2,e1);
        ans = 0 ; DFS_END(1,0);
        printf("%lld\n",ans);
    }
    return 0;
}