【算法】输入一棵二叉树，判断该二叉树是否是平衡二叉树。

public boolean IsBalanced_Solution(TreeNode root) {
        return getDepth(root) != -1;
    }  
    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) return -1;
        return Math.abs(left - right) > 1 ? -1 : 1 + Math.max(left, right);
    }