find-mode-in-binary-search-tree.cpp

// Time:  O(n)
// Space: O(h)

/**
 * 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) {
        if (!root) {
            return {};
        }
        
        vector<int> result;
        TreeNode *prev = nullptr;
        int cnt = 1, max_cnt = 0;
        inorder(root, &prev, &cnt, &max_cnt, &result);
        return result;
    }
    
private:    
    void inorder(TreeNode *root, TreeNode **prev, int *cnt, int *max_cnt, vector<int> *result) {
        if (root == nullptr) {
            return;
        }
        
        inorder(root->left, prev, cnt, max_cnt, result);
        if (*prev) {
            if (root->val == (*prev)->val) {
                ++(*cnt);
            } else {
                *cnt = 1;
            }
        }
        if (*cnt > *max_cnt) {
            *max_cnt = *cnt;
            result->clear();
            result->emplace_back(root->val);
        } else if (*cnt == *max_cnt) {
            result->emplace_back(root->val);
        }
        *prev = root;
        inorder(root->right, prev, cnt, max_cnt, result);
    }
};