LeetCode in Ruby: 102. Binary Tree Level Order Traversal

algobot76 profile image Kaitian Xie Updated on ・1 min read


def level_order(root)
  result = []
  return result if root.nil?

  queue = []
  queue << root

  until queue.empty?
    level_size = queue.length
    level = []
    level_size.times do
      node = queue.shift
      level << node.val
      queue << node.left unless node.left.nil?
      queue << node.right unless node.right.nil?
    result << level


In general, this approach is to go through each level of a given binary tree and add the val of each node to result. For each iteration, we first create an empty level array used to store the values of the nodes at the same level. Then we add the val of each node to the level array and push the nodes at the next level to queue if any. At the end of each iteration, we push the level array to result.

Time complexity: O(n)

Extra memory: O(n)


def level_order(root, result = [], level = 0)
  return result unless root

  result << [] if result.length == level
  result[level] << root.val
  level_order(root.left, result, level + 1)
  level_order(root.right, result, level + 1)

This is similar to the previous approach, except that this one is implemented using recursion. Interestingly, unlike the iterative approach, the recursive approach does not need a queue to temporarily store the nodes at the next level.

Time complexity: O(n)

Extra memory: O(1)


Editor guide

Can you explain me the following code? Mainly the <<... #RubyNewbie here

result << [] if result.length == level

It means that just before we begin to store the values of the next level, we add an empty array that will be used to store the values.