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.
For example:
Given binary tree [3,9,20,null,null,15,7], return its depth = 3.
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.
For example:
Given binary tree [3,9,20,null,null,15,7], return its depth = 3.