Example: Merge Sort Implementation in C++

// Merge Sort Implementation
#include <iostream> // For input/output operations
#include <vector>   // For using std::vector

// Function to merge two sorted subarrays
// arr[l..m] and arr[m+1..r] into arr[l..r]
void merge(std::vector<int>& arr, int left, int mid, int right) {
    int n1 = mid - left + 1; // Size of the left subarray
    int n2 = right - mid;    // Size of the right subarray

    // Create temporary arrays
    std::vector<int> L(n1);
    std::vector<int> R(n2);

    // Copy data to temp arrays L[] and R[]
    for (int i = 0; i < n1; i++)
        L[i] = arr[left + i];
    for (int j = 0; j < n2; j++)
        R[j] = arr[mid + 1 + j];

    // Merge the temp arrays back into arr[left..right]
    int i = 0; // Initial index of first subarray
    int j = 0; // Initial index of second subarray
    int k = left; // Initial index of merged subarray

    while (i < n1 && j < n2) {
        if (L[i] <= R[j]) {
            arr[k] = L[i];
            i++;
        } else {
            arr[k] = R[j];
            j++;
        }
        k++;
    }

    // Copy the remaining elements of L[], if any
    while (i < n1) {
        arr[k] = L[i];
        i++;
        k++;
    }

    // Copy the remaining elements of R[], if any
    while (j < n2) {
        arr[k] = R[j];
        j++;
        k++;
    }
}

// Main function that implements Merge Sort
// arr is the array to be sorted, left is the starting index, right is the ending index
void mergeSort(std::vector<int>& arr, int left, int right) {
    if (left >= right) { // Base case: if the array has 0 or 1 element, it's already sorted
        return;
    }
    int mid = left + (right - left) / 2; // Find the middle point to divide the array into two halves
    mergeSort(arr, left, mid);           // Recursively sort the first half
    mergeSort(arr, mid + 1, right);      // Recursively sort the second half
    merge(arr, left, mid, right);        // Merge the sorted halves
}

// Utility function to print an array
void printArray(const std::vector<int>& arr) {
    for (int x : arr) {
        std::cout << x << " ";
    }
    std::cout << std::endl;
}

int main() {
    // Step 1: Initialize an array
    std::vector<int> arr = {12, 11, 13, 5, 6, 7};
    int arr_size = arr.size();

    // Step 2: Print the original array
    std::cout << "Given array is \n";
    printArray(arr);

    // Step 3: Call mergeSort to sort the array
    mergeSort(arr, 0, arr_size - 1);

    // Step 4: Print the sorted array
    std::cout << "\nSorted array is \n";
    printArray(arr);

    // Test with another array
    std::vector<int> arr2 = {38, 27, 43, 3, 9, 82, 10};
    arr_size = arr2.size();
    std::cout << "\nGiven array 2 is \n";
    printArray(arr2);
    mergeSort(arr2, 0, arr_size - 1);
    std::cout << "\nSorted array 2 is \n";
    printArray(arr2);

    return 0;
}