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)

/**
 * 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) {
        vector<int> res;
        int mx = 0;
        unordered_map<int, int> m;
        inorder(root, m, mx);
        for(auto a : m)
        {
            if(a.second == mx)
                res.push_back(a.first);
        }
        return res;
    }
    
    void inorder(TreeNode * node, unordered_map<int, int> &m, int &mx)
    {
        if(!node) return;
        inorder(node->left, m, mx);
        mx = max(mx, ++m[node->val]);
        inorder(node->right, m, mx);
    }
};