Tree Painting CF-1187E（换根DP）

题意：

给出一棵树，选一个节点作为根，使得所有子树的和最大，输出最大值。

思路：

换根$DP$。

代码：

//#include<bits/stdc++.h>
#include <set>
#include <map>
#include <stack>
#include <cmath>
#include <queue>
#include <cstdio>
#include <string>
#include <vector>
#include <cstring>
#include <iostream>
#include <algorithm>

#define ll long long
#define pll pair<ll,ll>
#define pii pair<int,int>
#define bug printf("*********
")
#define FIN freopen("input.txt","r",stdin);
#define FON freopen("output.txt","w+",stdout);
#define IO ios::sync_with_stdio(false),cin.tie(0)
#define ls root<<1
#define rs root<<1|1
#define pb push_back

using namespace std;
const int inf = 2e9 + 7;
const ll Inf = 1e18 + 7;
const int maxn = 2e5 + 5;
const int mod = 1e9 + 7;

int n;
vector<int>vec[maxn];
int son[maxn];
ll sum = 0, ans = 0;;

void dfs1(int x, int fa)
{
    son[x] = 1;
    for (int i = 0, len = vec[x].size(); i < len; ++i)
    {
        int v = vec[x][i];
        if (v == fa)    continue;
        dfs1(v, x);
        son[x] += son[v];
    }
    sum += son[x];
    ans = max(ans, sum);
}

void dfs(int x, int fa, ll res)
{
    for (int i = 0, len = vec[x].size(); i < len; ++i)
    {
        int v = vec[x][i];
        if (v == fa)    continue;
        ll tmp = res - son[v] + n - son[v];
        ans = max(ans, tmp);
        dfs(v, x, tmp);
    }
}

int main()
{
    scanf("%d", &n);
    for (int i = 1; i < n; ++i)
    {
        int u, v;
        scanf("%d %d", &u, &v);
        vec[u].push_back(v);
        vec[v].push_back(u);
    }
    dfs1(1, 0);
    dfs(1, 0, sum);
    printf("%lld
", ans);
}