public int maxDepth(TreeNode root) {
		if(root == null){
			return 0;
		}
		if(root.left == null && root.right == null){//无子树
			return 1;
		}
		int leftDepth = maxDepth(root.left);
		int rightDepth = maxDepth(root.right);
		int max = leftDepth > rightDepth ? leftDepth : rightDepth;
		return max +1;
	}