Problem

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

Example:

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

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
// solution 1: recursion
// Time O(n)
// Space O(h)
class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
        
        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}

// Todo: iterative