501. Find Mode in Binary Search Tree.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
    private int max = 0;
    
    public int[] findMode(TreeNode root) {
        
        if(root==null) return new int[0];
        
        helper(root);
        
        List<Integer> list = new LinkedList<>();
        for(int key: map.keySet()){
            if(map.get(key) == max) list.add(key);
        }
        
        int[] res = new int[list.size()];
        for(int i = 0; i<res.length; i++) res[i] = list.get(i);
        return res;
    }
    
    public void helper(TreeNode node){
        map.put(node.val, map.getOrDefault(node.val, 0)+1);
        max = Math.max(map.get(node.val), max);
        
        if(node.left!=null){
            helper(node.left);
        }
        
        if(node.right!=null){
            helper(node.right);
        }
        
    }
    
}

This solution used a Hash map, which requires additional spaces and is not allowed by the question, LeetCode 501. Find Mode in Binary Search Tree.