Convert LeetCode Array Binary Tree To Tree Structure

buildBinaryTreeFromArray.js

// Definition for a binary tree node.
function TreeNode(val, left, right) {
  this.val = val === undefined ? 0 : val;
  this.left = left === undefined ? null : left;
  this.right = right === undefined ? null : right;
}

/**
 * @param {Array<Number>} nodes
 * @return {null | TreeNode}
 */
function buildBinaryTreeFromArray(nodes) {
  if (nodes.length === 0) return null;

  const root = new TreeNode(nodes[0]);

  function recurse(node, index) {
    if (!node || index === nodes.length - 1) return;
    const leftIndex = 2 * index + 1;
    const rightIndex = 2 * index + 2;
    node.left = nodes[leftIndex] ? new TreeNode(nodes[leftIndex]) : null;
    node.right = nodes[rightIndex] ? new TreeNode(nodes[rightIndex]) : null;

    recurse(node.left, leftIndex);
    recurse(node.right, rightIndex);
  }

  recurse(root, 0);

  return root;
}

const t = buildBinaryTreeFromArray([1, 2, 3, null, 4]);
console.log(t);

//       1
//     /  \
//   /     \
// 2         3
//   \
//     \
//       4