[LC] 39. Combination Sum

Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

The same repeated number may be chosen from candidates unlimited number of times.

Note:

• All numbers (including target) will be positive integers.
• The solution set must not contain duplicate combinations.

Example 1:

Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
  [7],
  [2,2,3]
]

Example 2:

Input: candidates = [2,3,5], target = 8,
A solution set is:
[
  [2,2,2,2],
  [2,3,3],
  [3,5]
]

Time: O(N^|target|)
Space: O(|target|)

 class Solution:
     def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
         res = []
         lst = []
         self.dfs(candidates, lst, res, 0, target)
         return res

     def dfs(self, candidates, lst, res, start, reminder):
         if reminder < 0:
             return
         if reminder == 0:
             res.append(list(lst))
             return
         for i in range(start, len(candidates)):
             cur_num = candidates[i]
             lst.append(cur_num)
             self.dfs(candidates, lst, res, i, reminder - cur_num)
             lst.pop()

class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (candidates == null) {
            return result;
        }
        List<Integer> list = new ArrayList<>();
        helper(candidates, list, 0, target, result);
        return result;
    }

    private void helper(int[] nums, List<Integer> list, int index, int reminder, List<List<Integer>> result)     {
        if (reminder < 0) {
            return;
        }
        if (reminder == 0) {
            result.add(new ArrayList<>(list));
            return;
        }
        for (int i = index; i < nums.length; i++) {
            list.add(nums[i]);
            helper(nums, list, i, reminder - nums[i], result);
            list.remove(list.size() - 1);
        }
    }
}