输入一棵二叉树，判断该二叉树是否是平衡二叉树。

如果一棵树是平衡二叉树则返回该树的高度，否则返回-1
对于一棵树只要判断其左右子树均为平衡二叉树且高度相差不超过1则为平衡二叉树

public class Solution {
    public boolean IsBalanced_Solution(TreeNode root) {
        int h = getDepth(root);
        boolean res = h == -1 ? false : true ;
        return res;
    } 

    private int getDepth(TreeNode root){
        if( root == null) return 0;
        int left = getDepth(root.left);
        if(left == -1) return -1;
        int right = getDepth(root.right);
        if(right == -1 || Math.abs(left-right) > 1)
            return -1;
        else return Math.max(left, right) + 1;
    }
}