Leetcode501. Find Mode in Binary Search Tree大道至简

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 findMode(TreeNode* root) {
        unordered_map map;
        vector vi;
        findMode(root, map);
        int max = 0;
        for (auto it = map.begin(); it != map.end(); it++)
        {
            if (it->second > max) {
                max = it->second;
            }
        }
        for (auto it = map.begin(); it != map.end(); it++)
        {
            if (it->second == max) {
                vi.push_back(it->first);
            }
        }
        return vi;
    }

    void findMode(TreeNode* root, unordered_map& map) {
        if (root == nullptr) {
            return;
        }
        map[root->val]++;
        if (root->left) {
            findMode(root->left, map);
        }
        if (root->right) {
            findMode(root->right, map);
        }
    }
};