Introduction to Tree Breadth First Search Pattern

You are given a binary tree. Return its values one level at a time.

        1
      /   \
     2     3          becomes   [[1], [2, 3], [4, 5, 6]]
    / \     \
   4   5     6

The root is level zero. Its children form the next level. Their children form the level after that.

A normal recursive traversal often goes deep into one branch first. It may visit node 4 before node 3. That order does not match the required output.

We need to visit all nodes at one depth before visiting a deeper node.

A queue provides this order. A queue follows First In, First Out order