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, … , a_k) must be in non-descending order. (ie, a₁ â‰¤ a₂ â‰¤ … ≤ a_k).
• 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] 

Solution:

01 public ArrayList<ArrayList<Integer>> combinationSum(int[] candidates, int target) {
02     ArrayList<ArrayList<Integer>> results=new ArrayList<ArrayList<Integer>>();
03     if(candidates.length==0)
04         return results;
05     ArrayList<Integer> temp=new ArrayList<Integer>();
06     Arrays.sort(candidates);
07     doCombine(results, temp, candidates, 0, target);
08     return results;
09   
10 }
11 
12 public void doCombine(ArrayList<ArrayList<Integer>> results, ArrayList<Integer> temp, int [] candidates, int start, int target)
13 {
14   //Iterate through each number. Start from the first number.
15   //Each number can be used multiple times, until we pick up the next number
16     for(int i=start;i<candidates.length;i++)
17     {
18       //Base case: target equals the current number, add it to the temp buffer, output it.
19         if(target-candidates[i]==0)
20         {
21             temp.add(candidates[i]);
22             ArrayList<Integer> out=new ArrayList<Integer>(temp);
23             results.add(out);
24           
25         }
26       //Target larger than the current number, then new target becomes target-candidates[i].
27       //Add candidates[i] to the temp buffer. Call the fuction itself recursively
28         else if(target-candidates[i]>0)
29         {
30               
31             temp.add(candidates[i]);
32             doCombine(results, temp, candidates, i, target-candidates[i]);
33         }
34 
35       //Target smaller than the current number. Such combination is not valid. Return.
36         else
37         {
38             return;
39         }
40        //Remove the last element of the temporary buffer
41         int size=temp.size();
42         if(size>0)
43             temp.remove(size-1);
44     }
45 }