[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 + 1, 0);
    for (int i = 0; i < (int)nums.size(); i++)
        counts[nums[i]]++;
    for (int i = 1; i <= upper; i++)
        counts[i] += counts[i - 1];
    vector<int> sorted(nums.size());
    for (int i = (int)nums.size() - 1; i >= 0; i--) {
        sorted[counts[nums[i]] - 1] = nums[i];
        counts[nums[i]]--;
    }
    swap(nums, sorted);
}

void print(vector<int>& nums) {
    for (int i = 0; i < (int)nums.size(); i++)
        printf("%d ", nums[i]);
    printf("
");
}

void countingSortTest(int len, int upper) {
    vector<int> nums(len);
    srand((unsigned)time(NULL));
    for (int i = 0; i < len; i++)
        nums[i] = rand() % (upper + 1);
    print(nums);
    countingSort(nums, upper);
    print(nums);
}

int main(void) {
    countingSortTest(20, 5);
    system("pause");
    return 0;
}

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

2 3 4 1 5 1 1 5 4 0 3 2 2 5 5 3 5 5 3 4
0 1 1 1 2 2 2 3 3 3 3 4 4 4 5 5 5 5 5 5