Example: Merge Sort with User Input in C++

// Merge Sort with User Input
#include <iostream> // For input/output operations
#include <vector>   // For dynamic array (vector)

// Function to merge two sorted sub-arrays
void merge(std::vector<int>& arr, int left, int mid, int right) {
    int n1 = mid - left + 1; // Size of the left sub-array
    int n2 = right - mid;    // Size of the right sub-array

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

    // Copy data to temporary 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 temporary arrays back into arr[left...right]
    int i = 0; // Initial index of first sub-array
    int j = 0; // Initial index of second sub-array
    int k = left; // Initial index of merged sub-array

    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++;
    }
}

// Function to perform Merge Sort
void mergeSort(std::vector<int>& arr, int left, int right) {
    if (left >= right) { // Base case: array with 0 or 1 element is already sorted
        return;
    }
    int mid = left + (right - left) / 2; // Calculate midpoint to avoid overflow
    mergeSort(arr, left, mid);           // Sort first half
    mergeSort(arr, mid + 1, right);      // Sort second half
    merge(arr, left, mid, right);        // Merge the sorted halves
}

int main() {
    // Step 1: Get array size from user
    int size;
    std::cout << "Enter the number of elements: ";
    std::cin >> size;

    if (size <= 0) {
        std::cout << "Number of elements must be positive." << std::endl;
        return 1; // Indicate an error
    }

    // Step 2: Get array elements from user
    std::vector<int> arr(size);
    std::cout << "Enter " << size << " integers:" << std::endl;
    for (int i = 0; i < size; ++i) {
        std::cout << "Element " << i + 1 << ": ";
        std::cin >> arr[i];
    }

    // Step 3: Display the original array
    std::cout << "\nOriginal array: ";
    for (int x : arr) {
        std::cout << x << " ";
    }
    std::cout << std::endl;

    // Step 4: Perform Merge Sort
    mergeSort(arr, 0, size - 1);

    // Step 5: Display the sorted array
    std::cout << "Sorted array: ";
    for (int x : arr) {
        std::cout << x << " ";
    }
    std::cout << std::endl;

    return 0;
}