counting_sort.cpp: Add Counting Sort Algorithm

This adds Counting sort which sorts array by counting the
occurences of elements.

Closes #299

Author: Paawan Mukker

README.md

@@ -16,7 +16,7 @@ This repository contains examples of various algorithms written on different pro
 | [QuickSort](https://en.wikipedia.org/wiki/Quicksort)                                            |                                       |                                       |                                       | [:octocat:](quicksort/Python)         |
 | [Merge Sort](https://en.wikipedia.org/wiki/Merge_sort)                                          |                                       |                                       |                                       | [:octocat:](merge_sort/Python)        |
 | [Insertion Sort](https://en.wikipedia.org/wiki/Insertion_sort)                                  | [:octocat:](insertion_sort/C)         | [:octocat:](insertion_sort/Cpp)       |                                       | [:octocat:](insertion_sort/Python)    |
-| [Counting Sort](https://en.wikipedia.org/wiki/Counting_sort)                                    |                                       |                                       |                                       | [:octocat:](counting_sort/Python)     |
+| [Counting Sort](https://en.wikipedia.org/wiki/Counting_sort)                                    |                                       | [:octocat:](counting_sort/Cpp)                                       |                                       | [:octocat:](counting_sort/Python)     |
 | [Radix Sort](https://en.wikipedia.org/wiki/Radix_sort)                                          |                                       |                                       |                                       | [:octocat:](radix_sort/Python)        |
 | [Binary Search](https://en.wikipedia.org/wiki/Binary_search_algorithm)                          |                                       | [:octocat:](binary_search/Cpp)        |                                       | [:octocat:](binary_search/Python)     |
 | [Bubble Sort](https://en.wikipedia.org/wiki/Bubble_sort)                                        | [:octocat:](bubble_sort/C)            | [:octocat:](bubble_sort/Cpp)          |                                       |                                       |

counting_sort/Cpp/counting_sort.cpp

@@ -0,0 +1,73 @@
+#include <iostream>
+#include <algorithm>
+#include <cmath>
+using namespace std;
+
+/*
+Counting sort implementation for sorting given array.
+Note - Works on Non-negative integer array only.
+
+Parameters:
+int a[] - Integer array a to be sorted.
+int n	- size of array a.
+int r.	- Range of numbers in a.
+*/
+
+void counting_sort(int a[], int n, int r) {
+	if(a == NULL) return;
+
+	int *cnt = new int[r+1]; //array to store count of each occurences
+	int *out = new int[n]; //Out array to store sorted values
+
+
+	// Intitialise the count array to 0
+	for (int i = 0; i <= r; ++i)
+		cnt[i] = 0;
+
+	// Count each occurences digit wise
+	for (int i = 0; i < n; ++i)
+		cnt[a[i]]++;
+
+	// Cumulative count array
+	for (int i = 1; i <= r; ++i)
+		cnt[i]+=cnt[i-1];
+
+	// Sort
+	for (int i = n - 1; i >= 0; i--) {
+		// Note - starting from n-1 to maintain "stable sort" property
+		out[cnt[a[i]] - 1] = a[i];
+		cnt[a[i]]--;
+	}
+
+	// Copy results back to orignal array
+	for (int i = 0; i < n; i++)
+		a[i] = out[i];
+}
+
+
+int main() {
+	ios_base::sync_with_stdio(false); //For faster io
+	int a[] = {1, 2, 9, 293, 41, 15, 52, 3, 121, 7, 1223, 3449, 15, 1 };
+	int n = sizeof(a)/sizeof(a[0]);
+
+	// Get the iterator to max element for calculating range of number
+	int * max_a_itr = max_element(a, a+n);
+	int max_a = *max_a_itr;
+
+
+	cout << "Input Array:\n";
+	for (int i = 0; i < n; i++) {
+		cout << a[i] << " ";
+	}
+	cout << "\n";
+
+	counting_sort(a, n, max_a);
+
+	cout << "Sorted Array:\n";
+	for (int i = 0; i < n; i++) {
+		cout << a[i] << " ";
+	}
+	cout << "\n";
+
+	return 0;
+}