Find Mode in Binary Search Tree LeetCode solution in JavaScript

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/**
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 * Definition for a binary tree node.
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 * function TreeNode(val, left, right) {
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 *     this.val = (val===undefined ? 0 : val)
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 *     this.left = (left===undefined ? null : left)
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 *     this.right = (right===undefined ? null : right)
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 * }
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 */
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/**
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 * @param {TreeNode} root
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 * @return {number[]}
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 */
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var findMode = function (root) {
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  const dup = new Map();
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  helper(root, dup);
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  let res = [];
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  let max = Number.MIN_VALUE;
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  for (const [key, value] of dup) {
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    if (value > 1 && value >= max) {
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      res.push(key);
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      max = Math.max(max, value);
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    }
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  }
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  return res.length > 0 ? res : [...dup.keys()];
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};
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function helper(root, dup) {
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  if (root !== null) {
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    dup.set(root.val, ~~dup.get(root.val) + 1);
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    helper(root.left, dup);
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    helper(root.right, dup);
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  }
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}