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.

中文：给定一棵二叉树，检测其高度上平衡否。

于此题，一棵高度上平衡的二叉树被定义为一棵每个节点上的两个子树上的深度差不能大于1的二叉树.

递归判断。

Java：

	public boolean isBalanced(TreeNode root) {
		if (root == null)
			return true;
		if (root.left == null && root.right == null)
			return true;
		if (Math.abs(depth(root.left) - depth(root.right)) > 1)
			return false;
		return isBalanced(root.left) && isBalanced(root.right);
	}

	public int depth(TreeNode root) {
		if (root == null)
			return 0;
		return 1 + Math.max(depth(root.left), depth(root.right));
	}