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]
]

class Solution {
    public static List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> ans = new ArrayList<>();
        backTrack(ans, new ArrayList<Integer>(), candidates, target, 0);
        return ans;
    }

    public static void backTrack(List<List<Integer>> ans, ArrayList<Integer> temp, int[] candidates, int target,
            int start) {
        if (target <= 0) {
            if (target == 0) {
                ans.add(new ArrayList<>(temp));
            }
            return;
        }
        for (int i = start; i < candidates.length; i++) {
            temp.add(candidates[i]);
            backTrack(ans, temp, candidates, target - candidates[i], i);
            temp.remove(temp.size() - 1);
        }
    }
}