Given a binary tree, find its maximum depth. The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node. Note: A leaf is a node with no children.

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

Given a binary tree, return the inorder traversal of its nodes’ values.

Given a binary tree, return the postorder traversal of its nodes’ values.

Given a binary tree, return the preorder traversal of its nodes’ values.