leetcode二叉树问题

Example:

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

    3
   / 
  9  20
    /  
   15   7

return its depth = 3.

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