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：

 1 //#include<bits/stdc++.h>
 2 #include <set>
 3 #include <map>
 4 #include <stack>
 5 #include <cmath>
 6 #include <queue>
 7 #include <cstdio>
 8 #include <string>
 9 #include <vector>
10 #include <cstring>
11 #include <iostream>
12 #include <algorithm>
13 
14 #define ll long long
15 #define pll pair<ll,ll>
16 #define pii pair<int,int>
17 #define bug printf("*********\n")
18 #define FIN freopen("input.txt","r",stdin);
19 #define FON freopen("output.txt","w+",stdout);
20 #define IO ios::sync_with_stdio(false),cin.tie(0)
21 #define ls root<<1
22 #define rs root<<1|1
23 #define pb push_back
24 
25 using namespace std;
26 const int inf = 2e9 + 7;
27 const ll Inf = 1e18 + 7;
28 const int maxn = 2e5 + 5;
29 const int mod = 1e9 + 7;
30 
31 int n;
32 vector<int>vec[maxn];
33 int son[maxn];
34 ll sum = 0, ans = 0;;
35 
36 void dfs1(int x, int fa)
37 {
38     son[x] = 1;
39     for (int i = 0, len = vec[x].size(); i < len; ++i)
40     {
41         int v = vec[x][i];
42         if (v == fa)    continue;
43         dfs1(v, x);
44         son[x] += son[v];
45     }
46     sum += son[x];
47     ans = max(ans, sum);
48 }
49 
50 void dfs(int x, int fa, ll res)
51 {
52     for (int i = 0, len = vec[x].size(); i < len; ++i)
53     {
54         int v = vec[x][i];
55         if (v == fa)    continue;
56         ll tmp = res - son[v] + n - son[v];
57         ans = max(ans, tmp);
58         dfs(v, x, tmp);
59     }
60 }
61 
62 int main()
63 {
64     scanf("%d", &n);
65     for (int i = 1; i < n; ++i)
66     {
67         int u, v;
68         scanf("%d %d", &u, &v);
69         vec[u].push_back(v);
70         vec[v].push_back(u);
71     }
72     dfs1(1, 0);
73     dfs(1, 0, sum);
74     printf("%lld\n", ans);
75 }