#501 - Find Mode in Binary Search Tree

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Problem Description

Given the root of a binary search tree (BST) with duplicates, return all the mode(s) (i.e., the most frequently occurred element) in it.

If the tree has more than one mode, return them in any order.

Solution

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var findMode = function(root) {
  const map = new Map();
  dfs(root);

  function dfs(root) {
    if (!root) return 0;
    map.set(root.val, (map.get(root.val) || 0) + 1);
    [root.left, root.right].forEach(dfs);
  }

  const max = Math.max(...map.values());
  return Array.from(map.keys()).filter(key => map.get(key) === max);
};