function compare(a, b) {
  if (a is less than b by some ordering criterion) {
    return -1; // a < b，a 在 b 前面
  }
  if (a is greater than b by the ordering criterion) {
    return 1; // a > b，a 在 b 后面
  }
  // a must be equal to b
  return 0; // a === b，并列排序，保持在源数组中的先后顺序
}