Sort in C++

MergeSort

#include <iostream>
#include <vector>

using namespace std;

namespace show {

template <typename T>
void show(vector<T> data, bool ln = true) {
    cout << '[';
    for (auto item : data) cout << item << ", ";
    cout << ']';
    if (ln) cout << endl;
}

}  // namespace show

namespace algorithm {

template <typename T>
vector<T> merge(const vector<T>& arr1, const vector<T> arr2) {
    vector<T> result;
    int i = 0, j = 0;
    while (i < arr1.size() && j < arr2.size())
        if (arr1[i] <= arr2[j])
            result.push_back(arr1[i++]);
        else
            result.push_back(arr2[j++]);

    while (i < arr1.size()) result.push_back(arr1[i++]);
    while (j < arr2.size()) result.push_back(arr2[j++]);

    return result;
}

template <typename T>
vector<T> mergesort(const vector<T>& arr, int lo = 0, int hi = -1) {
    if (hi == -1) hi = arr.size();
    if (hi - lo <= 1) return arr;
    int mid = (hi + lo) / 2;
    vector<T> left(arr.begin() + lo, arr.begin() + mid), right(arr.begin() + mid, arr.begin() + hi);
    vector<T> sorted_left = mergesort(left), sorted_right = mergesort(right);
    return merge(sorted_left, sorted_right);
}

template <typename T>  // inplace sort
void quicksort(vector<T>& arr, int lo = 0, int hi = -1) {
    if (hi == -1) hi = arr.size();
    if (hi - lo <= 1) return;
    int lo_ = lo, hi_ = hi;
    hi--;
    T pivot = arr[lo];
    while (lo < hi) {
        while (lo < hi && arr[hi] >= pivot) hi--;
        arr[lo] = arr[hi];
        while (lo < hi && arr[lo] <= pivot) lo++;
        arr[hi] = arr[lo];
    }
    arr[hi++] = pivot;
    quicksort(arr, lo_, lo);
    quicksort(arr, hi, hi_);
}

}  // namespace algorithm

int main() {
    vector<int> arr{9, 1, 4, 2, 3, 8, 4, 1, 8, 4, 6};
    cout << "Original data: ";
    show::show(arr);
    cout << "MergeSort: ";
    show::show(algorithm::mergesort(arr));
    cout << "QuickSort(inplace): ";
    algorithm::quicksort(arr);
    show::show(arr);
    return 0;
}

Result:

Original data: [9, 1, 4, 2, 3, 8, 4, 1, 8, 4, 6, ]
MergeSort: [1, 1, 2, 3, 4, 4, 4, 6, 8, 8, 9, ]
QuickSort(inplace): [1, 1, 2, 3, 4, 4, 4, 6, 8, 8, 9, ]