Check Completeness of a Binary Tree

Description

Given a binary tree, determine if it is a complete binary tree.

Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2^h nodes inclusive at the last level h.

 

Example 1:

Input: [1,2,3,4,5,6]
Output: true
Explanation: Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.

Example 2:

Input: [1,2,3,4,5,null,7]
Output: false
Explanation: The node with value 7 isn't as far left as possible.

 

Note:

1. The tree will have between 1 and 100 nodes.

Solution(javascript)

// 按层次遍历，看 null 之后是否还有非 null 元素
const isCompleteTree = function (root) {
  let current = [root]
  let hasNull = false
  while (current.length > 0) {
    const next = []
    for (const node of current) {
      if (node === null) {
        hasNull = true
      }
      if (node) {
        if (hasNull) {
          return false
        }
        next.push(node.left)
        next.push(node.right)
      }
    }
    current = next
  }
  return true
}