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] 

Java

public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> rst = new ArrayList<List<Integer>>();
        Arrays.sort(candidates);
        helper(rst, new ArrayList<Integer>(), candidates, 0, target);
        return rst;
    }

    private void helper(List<List<Integer>> rst, List<Integer> item, int[] candidates, int start, int target) {
        if(target < 0) {
            return;
        }

        if(target == 0) {
            rst.add(new ArrayList<Integer>(item));
            return;
        }

        for(int i = start; i < candidates.length; i++) {
            item.add(candidates[i]);
            helper(rst, item, candidates, i, target - candidates[i]);
            item.remove(item.size() - 1);
        }
    }
}