Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

此题和求二叉树的最短路径几乎一模一样。

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