content: package question0040_combination_sum_ii;
  
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.List;
  
  /**
   * 回溯。
   *
   * 执行用时6ms，击败77.19%。消耗内存：37.4MB，击败89.90%。
   */
  public class Solution {
      private List<List<Integer>> listList = new ArrayList<>();
  
      public List<List<Integer>> combinationSum2(int[] candidates, int target) {
          Arrays.sort(candidates);    //排序是关键
          combinationSum2(candidates, target, new ArrayList<>(), 0, 0);
          return listList;
      }
  
      private void combinationSum2(int[] candidates, int target, List<Integer> list, int sum, int begin) {
          if (sum >= target) {
              if (sum == target) {
                  listList.add(new ArrayList<>(list));
              }
              return;
          }
          for (int i = begin; i < candidates.length; i++) {
              if (i > begin && candidates[i] == candidates[i - 1]) {  //跳过非首次出现的数，避免重复
                  continue;
              }
              list.add(candidates[i]);
              combinationSum2(candidates, target, list, sum + candidates[i], i + 1);
              list.remove(list.size() - 1);
          }
      }
  }