Updated index.md: C++ implementation for count sort (#26609)

* Update index.md

Added a C++ implementation to count sort using std::vector

* fix: added code block syntax

guide/english/algorithms/sorting-algorithms/counting-sort/index.md

@@ -53,4 +53,21 @@ for (i=0; i < numbers.length; i++) {
 }
 ```
 
+### C++ Implementation
+```cpp
+#include <iostream>
 
+void countSort(int upperBound, int lowerBound, std::vector<int> numbersToSort) //lower and upper bounds of numbers in vector
+{
+  int range = upperBound - lowerBound;                  //create a range large enough to get every number between the min and max
+  std::vector<int> counts (range);                      //initialize of counts of the size of the range
+  std::fill(counts.begin(), counts.end(), 0);           //fill vector of zeros
+  
+  for (int i = 0; i < numbersToSort.size(); i++)
+  {
+      int index = numbersToSort[i] - lowerBound; //For example, if 5 is the lower bound and numbersToSort[i] is 5. index will be 0 and the       counts[index]+= 1;                         //count of 5 will be stored in counts[0]
+  }
+  
+  std::cout << counts << std::endl;
+} 
+```