组合总和 II

// 题目思路：
// 1. 使用Java Stream API来简化数据处理。
// 2. 由于Java Stream不太适合处理回溯问题，我们仍然需要辅助函数。
// 3. 排序、过滤重复项仍是避免重复组合的关键。
 
import java.util.List;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.stream.Collectors;
 
public class Solution {
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        return Arrays.stream(candidates).boxed()
                .sorted()
                .collect(Collectors.toList())
                .stream()
                .flatMap(candidate -> findCombinations(candidate, candidates, target))
                .distinct()
                .collect(Collectors.toList());
    }
 
    private Stream<List<Integer>> findCombinations(int candidate, int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<>();
        List<Integer> combination = new ArrayList<>();
        combination.add(candidate);
        if (candidate == target) {
            result.add(new ArrayList<>(combination));
        } else if (candidate < target) {
            for (int i = 1; i < candidates.length; i++) {
                if (candidates[i] == candidate) continue;
                int[] subCandidates = Arrays.copyOfRange(candidates, i, candidates.length);
                result.addAll(findCombinations(candidates[i], subCandidates, target - candidate));
            }
        }
        return result.stream();
    }
}