content: /*
   * Copyright 2024 Alibaba Group Holding Limited.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package com.alibaba.graphscope.common.ir.rel.metadata.glogue.utils;
  
  import java.util.ArrayList;
  import java.util.List;
  
  public class Combinations {
      public static <T> List<List<T>> getCombinations(List<T> elements, int k) {
          List<List<T>> results = new ArrayList<>();
          getCombinationsHelper(elements, k, 0, new ArrayList<>(), results);
          return results;
      }
  
      private static <T> void getCombinationsHelper(
              List<T> elements,
              int k,
              int startIndex,
              List<T> currentCombination,
              List<List<T>> results) {
          if (currentCombination.size() == k) {
              results.add(new ArrayList<>(currentCombination));
              return;
          }
          if (startIndex >= elements.size()
                  || elements.size() - startIndex < k - currentCombination.size()) {
              return;
          }
          currentCombination.add(elements.get(startIndex));
          getCombinationsHelper(elements, k, startIndex + 1, currentCombination, results);
          currentCombination.remove(currentCombination.size() - 1);
          getCombinationsHelper(elements, k, startIndex + 1, currentCombination, results);
      }
  }
  