Leetcode Balanced Binary Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isBalanced(TreeNode root) {
        if(root == null) {
            return true;
        }
        int diff = maxDepth(root.left) - maxDepth(root.right);
        if(diff > 1 || diff < -1 ) {
            return false;
        }
        return isBalanced(root.left) && isBalanced(root.right);
    }
    
    public int maxDepth(TreeNode root) {
        //use recursion
        if(root == null) {
            return 0;
        }
        int leftmax = maxDepth(root.left);
        int rightmax = maxDepth(root.right);
        return Math.max(leftmax, rightmax) + 1;   
    }
}