Thinking process

each element may only be used once

but there're duplicates

Mistakes

1. how to avoid duplicates answer?
   1. the number that is equal to its previous one but is larger than index

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

        //validate input
        if (candidates.length == 0) return result;
        Arrays.sort(candidates);
        findCombinationSum(candidates, target, result, new ArrayList<>(), 0);

        return result;
    }

    private void findCombinationSum(int[] candidates, int target, List<List<Integer>> result, 
                                        List<Integer> list, int index){
        //end conditions
        if (target < 0) {
            return;
        } else if (target == 0) {
            result.add(new ArrayList<>(list));
            return;
        }

        for (int i = index; i < candidates.length; ++i) {
            if (i > index && candidates[i] == candidates[i - 1]) continue;
            list.add(candidates[i]);
            findCombinationSum(candidates, target - candidates[i], result, list, i + 1);
            list.remove(list.size() - 1);
        }
    }
}