Flatten Binary Tree to Linked List 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 {void} Do not return anything, modify root in-place instead.
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 */
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var flatten = function (root) {
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  const dfs = (node) => {
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    if (!node) return;
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    if (!node.left && !node.right) return node;
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    const leftNode = node.left;
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    const rightNode = node.right;
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    const leftTree = dfs(leftNode);
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    const rightTree = dfs(rightNode);
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    if (leftTree) leftTree.right = rightNode;
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    node.left = null;
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    node.right = leftNode || rightNode;
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    return rightTree || leftTree;
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  };
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  dfs(root);
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  return root;
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};