gold-4-4

实现一个函数，检查二叉树是否平衡。在这个问题中，平衡树的定义如下：任意一个节点，其两棵子树的高度差不超过 1。

示例 1:

给定二叉树 [3,9,20,null,null,15,7]
    3
   / \
  9  20
    /  \
   15   7
返回 true 。

示例 2:

给定二叉树 [1,2,2,3,3,null,null,4,4]
      1
     / \
    2   2
   / \
  3   3
 / \
4 4
返回 false 。

//leetcode submit region begin(Prohibit modification and deletion)
/**
 * 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) {
        return balanceHeight(root) >= 0;
    }

    private int balanceHeight(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int leftH = balanceHeight(root.left);
        if (leftH == -1) {
            return -1;
        }
        int rightH = balanceHeight(root.right);
        if (rightH == -1) {
            return -1;
        }

        if (leftH - rightH > 1 || rightH - leftH > 1) {
            return -1;
        }

        return Math.max(leftH, rightH) + 1;
    }
}
//leetcode submit region end(Prohibit modification and deletion)