[Algorithms] Counting Sort

Counting sort is a linear time sorting algorithm. It is used when all the numbers fall in a fixed range. Then it counts the appearances of each number and simply rewrites the original array. For a nice introduction to counting sort, please refer to Introduction to Alogrithms, 3rd edition. The following code is basically a translation of the pseudo code there.

 #include <iostream>
 #include <vector>
 #include <ctime>

 using namespace std;

 // Counting Sort an Array with each element in [0, upper]
 void countingSort(vector<int>& nums, int upper) {
     vector<int> counts(upper + , );
     for (int i = ; i < (int)nums.size(); i++)
         counts[nums[i]]++;
     for (int i = ; i <= upper; i++)
         counts[i] += counts[i - ];
     vector<int> sorted(nums.size());
     for (int i = (int)nums.size() - ; i >= ; i--) {
         sorted[counts[nums[i]] - ] = nums[i];
         counts[nums[i]]--;
     }
     swap(nums, sorted);
 }

 void print(vector<int>& nums) {
     for (int i = ; i < (int)nums.size(); i++)
         printf("%d ", nums[i]);
     printf("\n");
 }

 void countingSortTest(int len, int upper) {
     vector<int> nums(len);
     srand((unsigned)time(NULL));
     for (int i = ; i < len; i++)
         nums[i] = rand() % (upper + );
     print(nums);
     countingSort(nums, upper);
     print(nums);
 }

 int main(void) {
     countingSortTest(, );
     system("pause");
     return ;
 }

If you run this program, you are expected to see (I run it on Microsoft Visual Studio Professional 2012):