Kth Smallest Element in a BST 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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 * @param {number} k
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 * @return {number}
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 */
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var kthSmallest = function (root, k) {
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  let stack = [];
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  while (root != null || stack.length > 0) {
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    while (root != null) {
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      stack.push(root);
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      root = root.left;
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    }
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    root = stack.pop();
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    if (--k == 0) break;
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    root = root.right;
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  }
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  return root.val;
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};