leetcode834 Sum of Distances in Tree

思路：

树形dp。

实现：

 class Solution
 {
 public:
     void dfs(int root, int p, vector<vector<int>>& G, vector<int>& cnt, vector<int>& res)
     {
         for (auto it: G[root])
         {
             if (it == p) continue;
             dfs(it, root, G, cnt, res);
             cnt[root] += cnt[it];
             res[root] += res[it] + cnt[it];
         }
         cnt[root]++;
     }
     void dfs2(int root, int p, vector<vector<int>>& G, vector<int>& cnt, vector<int>& res, int N)
     {
         for (auto it: G[root])
         {
             if (it == p) continue;
             res[it] = res[root] - cnt[it] + N - cnt[it];
             dfs2(it, root, G, cnt, res, N);
         }
     }
     vector<int> sumOfDistancesInTree(int N, vector<vector<int>>& edges)
     {
         vector<vector<int>> G(N, vector<int>());
         for (auto it: edges)
         {
             int a = it[], b = it[];
             G[a].push_back(b); G[b].push_back(a);
         }
         vector<int> cnt(N, ), res(N, );
         dfs(, -, G, cnt, res);
         dfs2(, -, G, cnt, res, N);
         return res;
     }
 }