110. 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.

题目含义：题意为判断一颗树是否为平衡二叉树。（PS：平衡二叉树的特性：它是一棵空树或它的左右两个子树的高度差的绝对值不超过1，并且左右两个子树都是一棵平衡二叉树）

    private int getDepth(TreeNode root) {
        if (root == null) return 0;
        return 1 + Math.max(getDepth(root.left), getDepth(root.right));
    }
    
    public boolean isBalanced(TreeNode root) {
        if (root == null) return true;
        if (Math.abs(getDepth(root.left) - getDepth(root.right)) > 1) return false;
        return isBalanced(root.left) && isBalanced(root.right);        
    }