数组排序的常规做法就是定义个比较函数然后传进排序函数：

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int main(int argc, char* argv[]) {
  vector<int> nums = {2, 3, 5, 7, 11, 13, 15, 17};
  sort(nums.begin(), nums.end(), [](int x, int y) {
    int units_x = x % 10;
    int units_y = y % 10;
    if (units_x == units_y) return x < y;
    return units_x < units_y;
  });
  for (int x : nums) cout << x << " ";
  cout << endl;
  return 0;
}

不过这里个位数的值只有0-9一共10种，所以像楼上一样分到10个vector中排序，然后依次汇总应该才是题目想要的做法，毕竟给定的数组是有序的，这个条件在之前的代码里没利用：

#include <assert.h>
#include <algorithm>
#include <array>
#include <iostream>
#include <vector>
using namespace std;

int main(int argc, char* argv[]) {
  // nums为有序数组
  vector<int> nums = {2, 3, 5, 7, 11, 13, 15, 17};
  array<vector<int>, 10> buckets;
  // TODO: use array<forward_list, 10> may be better?
  for (int x : nums) buckets[x % 10].emplace_back(x);
  size_t index = 0;
  for (const auto& v : buckets) {
    for (int x : v) {
      assert(index < nums.size());
      nums[index++] = x;
    }
  }
  for (int x : nums) cout << x << " ";
  cout << endl;
  return 0;
}