merge.cpp

#include <iostream>
#include <vector>

using namespace std;

// Merge two subarrays of arr[]
// First subarray is arr[l..m]
// Second subarray is arr[m+1..r]
void merge(vector<int>& arr, int l, int m, int r) {
    int n1 = m - l + 1;
    int n2 = r - m;

    // Create temp arrays
    vector<int> L(n1), R(n2);

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

    // Merge the temp arrays back into arr[l..r]
    int i = 0; // Initial index of first subarray
    int j = 0; // Initial index of second subarray
    int k = l; // 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 there are any
    while (i < n1) {
        arr[k] = L[i];
        i++;
        k++;
    }

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

// Main function that sorts arr[l..r] using merge()
void mergeSort(vector<int>& arr, int l, int r) {
    if (l >= r) {
        return; // Returns recursively
    }
    int m = l + (r - l) / 2;
    mergeSort(arr, l, m);
    mergeSort(arr, m + 1, r);
    merge(arr, l, m, r);
}

// Helper function to print an array
void printArray(const vector<int>& arr) {
    for (int i = 0; i < arr.size(); i++) {
        cout << arr[i] << " ";
    }
    cout << endl;
}

// Driver code
int main() {
    vector<int> arr = {12, 11, 13, 5, 6, 7};
    cout << "Given array is \n";
    printArray(arr);
    mergeSort(arr, 0, arr.size() - 1);
    cout << "\nSorted array is \n";
    printArray(arr);
    return 0;
}