Balanced Binary Tree

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    /**
     * 用一个INT 表示两种状态 当返回值大于等于0时 表示当前节点的深度，当返回值是-1时，表示不是平衡二叉树
     * @param root
     * @return
     */
    public int isBalancedHelper(TreeNode root) {
        if (root==null) {
            return 0;
        }
        int l = isBalancedHelper(root.left);
        int r = isBalancedHelper(root.right);
        if (l==-1||r==-1) {
            //左右子树有一个不是平衡二叉树，就不是平衡二叉树
            return -1;
        }
        if (Math.abs(l-r)>1) {
            //左右子树高度差大于1 不是平衡二叉树
            return -1;
        } else {
            return Math.max(l, r) + 1;
        }
    }

    /**
     * 平衡二叉树 左右子树是平衡二叉树 且 左右子树高度差小于等于1
     * @param root
     * @return
     */
    public boolean isBalanced(TreeNode root) {
        return isBalancedHelper(root)>=0;
    }
}