list2tree.js

function TreeNode(val) {
  this.val = val
  this.left = this.right = null
}

module.exports = function list2tree(list) {
  let tree = null
  let length = list.length
  let padding = 0
  let nodes = []
  for (let i = 0; i < length; ++i) {
    let val = list[i]
    let node = val ? new TreeNode(val) : null
    nodes[i + padding] = node
    if (i === 0) tree = node
    else {
      let p
      let fixed = 0
      while (p == null) {
        padding += fixed * 2
        index = Math.floor((i + padding - 1) / 2)
        p = nodes[index]
        fixed = 1
      }
      if (p) {
        p[(i & 1) ? 'left' : 'right'] = node
      }
    }
  }
  nodes.length = 0
  nodes = null
  return tree
}