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.

Example 1:

Given the following tree [3,9,20,null,null,15,7]:
Return true.
思路：如果从根结点开始，左右子树高度差>1时，就不是平衡树！

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private boolean result = true;
    public boolean isBalanced(TreeNode root) {
        maxDepth(root);
        return result;
    }
    public int maxDepth(TreeNode root){
        if(root == null) return 0;
        int l = maxDepth(root.left);
        int r = maxDepth(root.right);
        if(Math.abs(l-r)>1) result = false;
        return 1+Math.max(l,r);
    }
}