public int minDepth_helper(TreeNode root) {
		if (root == null)
			return Integer.MAX_VALUE;
		if (root.left == null && root.right == null)
			return 1;
		int left = minDepth_helper(root.left);
		int right = minDepth_helper(root.right);
		return Math.min(left, right) + 1;
	}