递归法输出一个不包含重复元素的数组的全部组合（简单直观的枚举，没有用DFS之类的。因为并不需要）

static int recurseCombinations(int array[], int arraySize, int combi[],
                               int combiSize, int arrayIdx, int combiEleIdx,
                               int ***combis, int *combiIdx) {
  // combination with size ‘combiSize‘ is selected
  if (combiEleIdx == combiSize) {
    if (*combis != NULL) {
      (*combis)[*combiIdx] = (int *)malloc(sizeof(int) * combiSize);
      for (int i = 0; i < combiSize; i++) {
        // printf("%d ", combi[i]);
        (*combis)[*combiIdx][i] = combi[i];
      }
      // printf("\n");
      *combiIdx = *combiIdx + 1;
    }
    return 1;
  }
  int combisCnt = 0;
  for (int i = arrayIdx; i < arraySize - (combiSize - combiEleIdx) + 1; i++) {
    combi[combiEleIdx] = array[i];
    combisCnt += recurseCombinations(array, arraySize, combi, combiSize, i + 1,
                                     combiEleIdx + 1, combis, combiIdx);
  }
  return combisCnt;
}

CombinationResults *combinations(int array[], int arraySize, int combiSize) {
  int **combis = NULL;
  int combiCnt = 0;

  int combiIdx = 0;
  int arrayIdx = 0;
  int combiEleIdx = 0;
  int *combi = (int *)calloc(combiSize, sizeof(int));
  combiCnt = recurseCombinations(array, arraySize, combi, combiSize, arrayIdx,
                                 combiEleIdx, &combis, &combiIdx);
  // printf("count of combinations: %d\n", combiCnt);
  combis = (int **)calloc(combiCnt, sizeof(int *));
  combiCnt = recurseCombinations(array, arraySize, combi, combiSize, arrayIdx,
                                 combiEleIdx, &combis, &combiIdx);

  free(combi);
  return combinationResults_init(combis, combiSize, combiCnt);
}

static void _test_Combinations() {
  int array[] = {0, 1, 2, 3, 4, 5};
  int arraySize = sizeof(array) / sizeof(int);
  for (int i = 1; i <= 6; i++) {
    CombinationResults *cr = combinations(array, arraySize, i);
    combinationResults_print(cr);
    combinationResults_destroy(cr);　　
  }
}