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

Maximum_Depth_of_Binary_Tree.java

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