https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
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],

    3
   / \
  9  20
    /  \
   15   7
 

return its depth = 3.
time: o(n): worst case linked list: n nodes, each take o(1)
space: o(n): worst case linked list: n nodes, each take one calling stack

1 public int maxDepth(TreeNode root) {
2         if (root == null) return 0 ;
3         int l = maxDepth(root.left);
4         int r = maxDepth(root.right) ;
 
5         return Math.max(l,r)+ 1;
6     }

LC.104. Maximum Depth of Binary Tree