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