BinaryTree recursive & iterative implementaion

BinaryTree.js

class TreeNode {
  constructor(val = 0, left = null, right = null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}

const buildTree = (arr) => {

  const build = (index) => {
    if (index >= arr.length || !arr[index]) return null
    const node = new TreeNode(arr[index])
    node.left = build(2 * index + 1)
    node.right = build(2 * index + 2)
    return node
  }

  return build(0)
}

console.log(buildTree([1, 2, 3, 4, 5, 6, 7]));

//-------------------------------------------------------------------------------
function buildBinaryTree(arr) {
  if (arr.length === 0) return null;

  const nodes = arr.map(val => val ? new TreeNode(val) : null);
  const n = nodes.length;

  for (let i = 0; i < n; i++) {
    if (nodes[i] !== null) {
      const leftIndex = 2 * i + 1;
      const rightIndex = 2 * i + 2;
      if (leftIndex < n) nodes[i].left = nodes[leftIndex];
      if (rightIndex < n) nodes[i].right = nodes[rightIndex];
    }
  }

  return nodes[0];
}

/*
       1
     /   \
    2     3
   / \   / \
  4   5 6   7
  
*/