LeetCode 39. Combination Sum

39. Combination Sum

Difficulty:: Medium

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]
]

Solution

Language: Java

class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (candidates == null || candidates.length == 0) {
            return result;
        }
        Arrays.sort(candidates);
        dfsHelper(candidates, 0, target, result, new ArrayList<>());
        return result;
    }
    
    private void dfsHelper(int[] candidates, int startIndex , int target, 
                           List<List<Integer>> result, List<Integer> combination) {
        if (target == 0) {
            result.add(new ArrayList<>(combination));
            return;
        }
        
        for (int i = startIndex; i < candidates.length; i++) {
            if (candidates[i] > target) {
                break;
            }
            combination.add(candidates[i]);
            dfsHelper(candidates, i, target - candidates[i], result, combination);
            combination.remove(combination.size() - 1);
        }
        
    }
}