MergeSort

#include <iostream>
#include <vector>
#include <ctime>
#include <cstdlib>


using namespace std;


void merge(vector<int>& v, int first1, int last1, int first2, int last2, vector<int>& tmp)
{
    int first = first1;
    int last = last2;
    int i = 0;
    while (first1 != last1 && first2 != last2
    {
        if (v[first1] < v[first2])
        {
            tmp[i] = v[first1];
            ++i;
            ++first1;
        }
        else
        {
            tmp[i] = v[first2];
            ++i;
            ++first2;
        }
    }
    while (first1 != last1)
    {
        tmp[i] = v[first1];
        ++i;
        ++first1;
    }
    while (first2 != last2)
    {
        tmp[i] = v[first2];
        ++i;
        ++first2;
    }
    i = 0;
    while (first != last)
    {
        v[first] = tmp[i];
        ++i;
        ++first;
    }
}

void mergesort(vector<int>& v, int first, int last)
{
    if (last - first <= 1)
        return;
    vector<int> tmp(v.size());
    mergesort(v, first, (first + last) / 2);
    mergesort(v, (first + last) / 2, last);
    merge(v, first, (first + last) / 2, (first + last) / 2, last, tmp);
}

int main()
{
    srand(time(NULL));
    vector<int> v(20);
    for (int i = 0; i < 20; ++i)
        v[i] = rand() % 100;
    for (int i = 0; i < 20; ++i)
        cout << v[i] << ' ';
    cout << '\n';
    mergesort(v, 0, 20);
    for (int i = 0; i < 20; ++i)
        cout << v[i] << ' ';
}