Maximum Width of 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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 * @return {number}
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 */
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var widthOfBinaryTree = function (root) {
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  if (!root) return 0;
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  let res = 0,
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    row = [[root, 0]];
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  while (row.length > 0) {
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    const nextRow = [];
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    let curStart = -1,
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      curEnd = -2;
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    for (let i = 0; i < row.length; i++) {
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      const [node, index] = row[i];
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      if (curStart === -1) {
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        curStart = index;
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      }
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      curEnd = index;
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      if (node.left) {
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        nextRow.push([node.left, index * 2]);
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      }
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      if (node.right) {
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        nextRow.push([node.right, index * 2 + 1]);
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      }
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    }
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    res = Math.max(res, curEnd - curStart + 1);
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    if (nextRow.length === 1) {
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      nextRow[0][1] = 0;
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    }
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    row = nextRow;
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  }
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  return res;
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};