text: // Hao Ren
  // 13 October, 2020
  // 1. Two Sum
  
  // Note: The returned array must be malloced, assume caller calls free().
  
  int *twoSum(int *nums, int numsSize, int target, int *returnSize) {
      *returnSize = 2;
      // Create an empty array by the size given. In this problem, the length of
      // returned array should be always "2".
      int *output = (int *)malloc((*returnSize) * sizeof(int));
      // Locate the first number to find if there is another one.
      for (int i = 0; i < numsSize; i++) {
          int diff = target - nums[i];
          for (int j = i + 1; j < numsSize; j++) {
              if (nums[j] == diff) {
                  // `i <= j;` should be always True.
                  output[0] = i;
                  output[1] = j;
              }
          }
      }
      // Return a pointer to the output array.
      return output;
  }
  