leetcode-501. Find Mode in Binary Search Tree

Given a binary search tree (BST) with duplicates, find all the mode(s) (the most frequently occurred element) in the given BST.

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than or equal to the node's key.
• The right subtree of a node contains only nodes with keys greater than or equal to the node's key.
• Both the left and right subtrees must also be binary search trees.

For example:
Given BST [1,null,2,2],

   1
    \
     2
    /
   2

return [2].

Note: If a tree has more than one mode, you can return them in any order.

Follow up: Could you do that without using any extra space? (Assume that the implicit stack space incurred due to recursion does not count).

思路：

1.递归记录每个节点，同时返回最大的相同节点数。

2.使用map 哈希思想是解决这种题目的最好选择

Accepted Code:

 /**
  * Definition for a binary tree node.
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     vector<int> findMode(TreeNode* root) {
         unordered_map<int,int> mp;
         vector<int> result;
         int modeCount=getModeCount(root,mp);

         for(pair<int,int> p:mp)
         {
             if(p.second==modeCount)
             result.push_back(p.first);
         }
         return result;
     }

     int getModeCount(TreeNode* root,unordered_map<int,int>& mp)
     {
         if(root==nullptr)
         return ;
         if(mp.find(root->val)==mp.end())
         {
             mp.insert(pair<int,int>(root->val,));
         }else
         {
             mp[root->val]++;
         }
         return max(mp[root->val],max(getModeCount(root->left,mp),getModeCount(root->right,mp)));
     }
 };