Find Elements in a Contaminated Binary 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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 */
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var FindElements = function (root) {
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  this.st = new Set();
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  recover = (root, val) => {
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    this.st.add(val);
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    if (root.left != null) recover(root.left, val * 2 + 1);
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    if (root.right != null) recover(root.right, val * 2 + 2);
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  };
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  recover(root, 0);
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};
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/**
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 * @param {number} target
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 * @return {boolean}
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 */
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FindElements.prototype.find = function (target) {
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  return this.st.has(target);
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};
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/**
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 * Your FindElements object will be instantiated and called as such:
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 * var obj = new FindElements(root)
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 * var param_1 = obj.find(target)
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 */