LeetCode #102 Binary Tree Level Order Traversal

Medium

Problem

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

Solution

By using a queue to do BFS approach from root, it’s easy.

Complexity

Because each node will be only visited once, it’s time complexity is O(n) if n denotes to count of nodes in this tree. And because the queue should have ability to save all leaves in this tree, it’s space complexity is O(n) as well.