回答編集履歴

  int shuffleArr[50];

  for(int i = 0;i < 50; i++) //shuffleArrの中に順に数字を入力する

  shuffleArr[i] = i;

  int b = rand() % i--;

  int tmp = shuffleArr[i]; // shuffleArr[i] と ShuffleArr[b]を交換

  shuffleArr[i] = shuffleArr[b];

  int randomNum = (gridSize * gridSize) / 2;

  for(int i = 0; i < randomNum ; i++)

  tmpArr[i] = tmpArr[i + randomNum] = shuffleArr[i];

  int b = rand() % i--;

  int tmp = tmpArr[i]; // tmpAddr[i] と tmpAddr[b]を交換

  tmpArr[i] = tmpArr[b];