mergesort.cpp

/* Merge SortTime complexity = nlogn */

#include<iostream>

using namespace std;

void merge(int arr[], int l, int mid, int r){

    int n1 = mid-l+1;
    int n2 = r-mid;

    int a[n1]; //we have stored elements from l to mid
    int b[n2]; //we have stored elements from mid to r
    //a and b are temporary arrays
    for (int i = 0; i < n1; i++)
    {
        a[i] = arr[l+i];
    }
    for (int i = 0; i < n2; i++)
    {
        b[i] = arr[mid+1+i];
    }
    
    int i=0, j=0, k=l;
    while (i<n1 && j<n2)
    {
        if (a[i]<b[j])
        {
            arr[k] = a[i];
            k++;
            i++;
        }
        else
        {
            arr[k] = b[j];
            k++;
            j++;
        }
        
    }
    
    while (i<n1)
    {
        arr[k] = a[i];
        k++;
        i++;
    }
    
    while (j<n2)
    {
        arr[k] = b[j];
        k++;
        j++;
    }

    
}

void mergeSort(int arr[], int l, int r){
    //l and r are pointers
    if (l<r)
    {
        int mid = (l+r)/2;
        mergeSort(arr, l, mid);
        mergeSort(arr, mid+1, r);

        merge(arr, l, mid, r);
    }
    
}

int main(){

    int arr[] = {5,4,3,2,1};
    mergeSort(arr, 0, 4);
    for (int i = 0; i < 5; i++)
    {
        cout<<arr[i]<<endl;
    }
    
    return 0;
}