Longest ZigZag Path in a Binary Tree LeetCode solution in JavaScript

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/** https://leetcode.com/problems/longest-zigzag-path-in-a-binary-tree/
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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 longestZigZag = function (root) {
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  this.out = 0;
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  // Recursive left and right node and find the largest height
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  let left = dfs(root.left, false) + 1;
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  let right = dfs(root.right, true) + 1;
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  this.out = Math.max(this.out, Math.max(left, right));
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  return this.out;
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};
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var dfs = function (node, isLeft) {
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  // Node is null, we return -1 because the caller will add 1, so result in 0 (no node visited)
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  if (node == null) {
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    return -1;
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  }
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  // No left or right node, we return 0 because the caller will add 1, so result in 1 (visited 1 node - this one)
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  if (node.left == null && node.right == null) {
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    return 0;
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  }
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  // Recursive to see which one is higher, zigzag to left or zigzag to right
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  let left = dfs(node.left, false) + 1;
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  let right = dfs(node.right, true) + 1;
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  this.out = Math.max(this.out, Math.max(left, right));
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  return isLeft === true ? left : right;
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};