Combination Sum

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]