Task: N-queens problem
C#: using System.Collections.Generic;
using static System.Linq.Enumerable;
using static System.Console;
using static System.Math;

namespace N_Queens
{
    static class Program
    {
        static void Main(string[] args)
        {
            var n = 8;
            var cols = Range(0, n);
            var combs = cols.Combinations(2).Select(pairs=> pairs.ToArray());
            var solved = from v in cols.Permutations().Select(p => p.ToArray())
                         where combs.All(c => Abs(v[c[0]] - v[c[1]]) != Abs(c[0] - c[1]))
                         select v;
            
            WriteLine($"{n}-queens has {solved.Count()} solutions");
            WriteLine("Position is row, value is column:-");
            var first = string.Join(" ", solved.First());
            WriteLine($"First Solution: {first}");
            Read();
        }

        
        public static IEnumerable<IEnumerable<T>> Permutations<T>(this IEnumerable<T> values)
        {
            if (values.Count() == 1)
                return values.ToSingleton();

            return values.SelectMany(v => Permutations(values.Except(v.ToSingleton())), (v, p) => p.Prepend(v));
        }

        public static IEnumerable<IEnumerable<T>> Combinations<T>(this IEnumerable<T> seq) =>
            seq.Aggregate(Empty<T>().ToSingleton(), (a, b) => a.Concat(a.Select(x => x.Append(b))));

        public static IEnumerable<IEnumerable<T>> Combinations<T>(this IEnumerable<T> seq, int numItems) =>
            seq.Combinations().Where(s => s.Count() == numItems);

        public static IEnumerable<T> ToSingleton<T>(this T item) { yield return item; }
    }
}

C++: 


#include <vector>
#include <iostream>
#include <iomanip>
#include <thread>
#include <future>



static void print(const std::vector<int> &pos)
{
	
	for (int i = 0; i < pos.size(); i++) {
		std::cout << std::setw(3) << char('a' + i);
	}

	std::cout << '\n';

	for (int row = 0; row < pos.size(); row++) {
		int col = pos[row];
		std::cout << row + 1 << std::setw(3 * col + 3) << " # ";
		std::cout << '\n';
	}

	std::cout << "\n\n";
}

static bool threatens(int row_a, int col_a, int row_b, int col_b)
{
	return row_a == row_b 
		or col_a == col_b 
		or std::abs(row_a - row_b) == std::abs(col_a - col_b); 
}




static bool good(const std::vector<int> &pos, int end_idx)
{
	for (int row_a = 0; row_a < end_idx; row_a++) {
		for (int row_b = row_a + 1; row_b < end_idx; row_b++) {
			int col_a = pos[row_a];
			int col_b = pos[row_b];
			if (threatens(row_a, col_a, row_b, col_b)) {
				return false;
			}
		}
	}

	return true;
}

static std::mutex print_count_mutex; 
static int n_sols = 0; 


static void n_queens(std::vector<int> &pos, int index)
{
	
	if (index >= pos.size()) {
		if (good(pos, index)) {
			std::lock_guard<std::mutex> lock(print_count_mutex);
			print(pos);
			n_sols++;
		}

		return;
	}

	
	if (not good(pos, index)) {
		return;
	}

	
	if (index == 0) {
		std::vector<std::future<void>> fts;
		for (int col = 0; col < pos.size(); col++) {
			pos[index] = col;
			auto ft = std::async(std::launch::async, [=]{ auto cpos(pos); n_queens(cpos, index + 1); });
			fts.push_back(std::move(ft));
		}

		for (const auto &ft : fts) {
			ft.wait();
		}
	} else { 
		for (int col = 0; col < pos.size(); col++) {
			pos[index] = col;
			n_queens(pos, index + 1);
		}
	}
}

int main()
{
	std::vector<int> start(12); 
	n_queens(start, 0);
	std::cout << n_sols << " solutions found.\n";
	return 0;
}