CombinationSum

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] 

 public List> combinationSum(int[] candidates, int target) {
List> ret=new ArrayList>();
if(candidates==null&&candidates.length==0){
return ret;
}
List path=new ArrayList<>();
Arrays.sort(candidates);
combinationSum(candidates, target,path,ret,0);
return ret;
}
public void combinationSum(int[] candidates, int target,
List path, List> ret, int index) {
// TODO 自动生成的方法存根
 if (target == 0) {
           // add the current set into the result.
           ret.add(new ArrayList(path));
           return;
       }
       
       if (target < 0) {
           return;
       }
       int len=candidates.length;
       for(int i=index;i         path.add(candidates[i]);
        combinationSum(candidates, target-candidates[i], path, ret, i);
        path.remove(path.size()-1);
       }
}