110. 平衡二叉树

给定二叉树 [3,9,20,null,null,15,7]

    3
   / \
  9  20
    /  \
   15   7
返回 true 。

给定二叉树 [1,2,2,3,3,null,null,4,4]

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4
返回 false 。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) {
            return true;
        }
        if (Math.abs(getTreeHeight(root.left) - getTreeHeight(root.right)) > 1) {
            return false;
        }
        return isBalanced(root.left) && isBalanced(root.right);
    }

    private int getTreeHeight(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return Math.max(getTreeHeight(root.left), getTreeHeight(root.right)) + 1;
    }
}