40. Combination Sum II

Given a collection of candidate numbers (candidates) and a target number (target), find all unique combinations in candidates where the candidate numbers sum to target.

Each number in candidates may only be used once in the combination.

Note: The solution set must not contain duplicate combinations.

Example 1:

Input: candidates = [10,1,2,7,6,1,5], target = 8
Output:
[
[1,1,6],
[1,2,5],
[1,7],
[2,6]
]

Example 2:

Input: candidates = [2,5,2,1,2], target = 5
Output:
[
[1,2,2],
[5]
]

Constraints:

• 1 <= candidates.length <= 100
• 1 <= candidates[i] <= 50
• 1 <= target <= 30

Solution:

The difference with 39, "Each number in candidates may only be used once in the combination."

class Solution {
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();

        List<Integer> subResult = new ArrayList<Integer>();

        int index = 0;

        Arrays.sort(candidates);

        helper(candidates, index, target, subResult, result);

        return result;
    }

    private void helper(int[] candidates, int index, int target, List<Integer> subResult, List<List<Integer>> result){
        if (target == 0){
            result.add(new ArrayList<>(subResult));
            return;
        }

        for (int i  = index; i < candidates.length; i++){
            if (i > index && candidates[i] == candidates[i-1]){
                continue;
            }

            if (candidates[i] > target){
                return;
            }

            subResult.add(candidates[i]);
            helper(candidates, i + 1, target - candidates[i], subResult, result);
            subResult.remove(subResult.size() -1);
        }
    }
}

// TC: O(branch^level) = O(2^n)
// SC: O(level) = O(n)