c_cpp 归并排序的非递归实现，慢于标准库

Posted 2021-05-14

#include <numeric>
#include <chrono>
#include <vector>
#include <random>
#include <iostream>
#include <algorithm>

using namespace std;

void merge(vector<int> &a, size_t l, size_t m, size_t u, vector<int> &aux) {
    auto p1 = l, p2 = m, pos = l;
    while (p1 != m && p2 != u)
        aux[pos++] = a[p2] < a[p1] ? a[p2++] : a[p1++];
    while (p1 != m) aux[pos++] = a[p1++];
    while (p2 != u) aux[pos++] = a[p2++];
    for (auto i = l; i < u; ++i) a[i] = aux[i];
}

void merge_sort(vector<int> &a) {
    auto n = a.size();
    auto aux = a;
    for (auto k = 1u; k < n; k <<= 1)
        for (auto u = 0u; u <= n;) {
            auto m = u + k;
            if (m >= n) break;
            u = min(m + k, n);
            merge(a, m - k, m, u, aux);
        }
}
int main() {
    vector<int> a(100'000);
    iota(a.begin(), a.end(), 0);
    random_device rd;
    mt19937 gen(rd());
    shuffle(a.begin(), a.end(), gen);
    auto t1 = chrono::system_clock::now();
    merge_sort(a);
    //stable_sort(a.begin(), a.end());
    auto t2 = chrono::system_clock::now();
    cout << (is_sorted(a.begin(), a.end()) ? "" : "Not ") << "Sorted" << endl;
    cout << "Time used: " << chrono::duration<double>(t2 - t1).count() << "s" << endl;
    return 0;
}