Combinations

	/**
	 * Given two integers n and k, return all possible combinations of k
numbers
	 * out of 1 ... n.
	 * 
	 * For example, If n = 4 and k = 2, a solution is:
	 * 
	 * 
	 * [ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]
	 * */