import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
/**
 * 39. Combination Sum
 * Given a set of candidate numbers (C) (without duplicates) 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.
 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]
 ]

 time : O(2^n)
 space : O(n)
 * @param candidates
 * @param target
 * @return
 */

public class CombinationSum {
	public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> res = new ArrayList<>();
        if (candidates == null || candidates.length == 0) return res;
        helper(res, new ArrayList<>(), candidates, target, 0);
        return res;
    }

    public void helper(List<List<Integer>> res, List<Integer> list, int[] candidates, int target, int start) {
        if (target == 0) {
            res.add(new ArrayList<>(list));
            return;
        }
        if(target<0)return;
        for (int i = start; i < candidates.length; i++) {
            list.add(candidates[i]);
            helper(res, list, candidates, target - candidates[i], i);
            list.remove(list.size() - 1);
        }
    }
    public static void main(String[] args) {
		// TODO Auto-generated method stub
    	int nums[]= {9,2,3,6,7};
    	
        CombinationSum one=new CombinationSum();
        one.combinationSum(nums,7);
	}
}