// Given a set of candidate numbers (C) and a target number (T),
// find all unique combinations in C where the candidate numbers sums to T.
//
// The same repeated number may be chosen from C unlimited number of times.
//
// Note:
// All numbers (including target) will be positive integers.
// Elements in a combination (a1, a2, … ,ak) must be in non-descending
order. (ie, a1 ≤ a2 ≤ … ≤ ak).
// The solution set must not contain duplicate combinations.
// For example, given candidate set 2,3,6,7 and target 7,
// A solution set is:
// [7]
// [2, 2, 3]