Task: Stable marriage problem
C#: using System;
using System.Collections.Generic;

namespace StableMarriage
{
    class Person
    {
        private int _candidateIndex;
        public string Name { get; set; }
        public List<Person> Prefs { get; set; }
        public Person Fiance { get; set; }
        
        public Person(string name) {
            Name = name;
            Prefs = null;
            Fiance = null;
            _candidateIndex = 0;
        }
        public bool Prefers(Person p) {
            return Prefs.FindIndex(o => o == p) < Prefs.FindIndex(o => o == Fiance);
        }
        public Person NextCandidateNotYetProposedTo() {
            if (_candidateIndex >= Prefs.Count) return null;
            return Prefs[_candidateIndex++];
        }
        public void EngageTo(Person p) {
            if (p.Fiance != null) p.Fiance.Fiance = null;
            p.Fiance = this;
            if (Fiance != null) Fiance.Fiance = null;
            Fiance = p;
        }
    }
    
    static class MainClass
    {
        static public bool IsStable(List<Person> men) {
            List<Person> women = men[0].Prefs;
            foreach (Person guy in men) {
                foreach (Person gal in women) {
                    if (guy.Prefers(gal) && gal.Prefers(guy))
                        return false;
                }
            }
            return true;
        }
        
        static void DoMarriage() {
            Person abe  = new Person("abe");
            Person bob  = new Person("bob");
            Person col  = new Person("col");
            Person dan  = new Person("dan");
            Person ed   = new Person("ed");
            Person fred = new Person("fred");
            Person gav  = new Person("gav");
            Person hal  = new Person("hal");
            Person ian  = new Person("ian");
            Person jon  = new Person("jon");
            Person abi  = new Person("abi");
            Person bea  = new Person("bea");
            Person cath = new Person("cath");
            Person dee  = new Person("dee");
            Person eve  = new Person("eve");
            Person fay  = new Person("fay");
            Person gay  = new Person("gay");
            Person hope = new Person("hope");
            Person ivy  = new Person("ivy");
            Person jan  = new Person("jan");
            
            abe.Prefs  = new List<Person>() {abi, eve, cath, ivy, jan, dee, fay, bea, hope, gay};
            bob.Prefs  = new List<Person>() {cath, hope, abi, dee, eve, fay, bea, jan, ivy, gay};
            col.Prefs  = new List<Person>() {hope, eve, abi, dee, bea, fay, ivy, gay, cath, jan};
            dan.Prefs  = new List<Person>() {ivy, fay, dee, gay, hope, eve, jan, bea, cath, abi};
            ed.Prefs   = new List<Person>() {jan, dee, bea, cath, fay, eve, abi, ivy, hope, gay};
            fred.Prefs = new List<Person>() {bea, abi, dee, gay, eve, ivy, cath, jan, hope, fay};
            gav.Prefs  = new List<Person>() {gay, eve, ivy, bea, cath, abi, dee, hope, jan, fay};
            hal.Prefs  = new List<Person>() {abi, eve, hope, fay, ivy, cath, jan, bea, gay, dee};
            ian.Prefs  = new List<Person>() {hope, cath, dee, gay, bea, abi, fay, ivy, jan, eve};
            jon.Prefs  = new List<Person>() {abi, fay, jan, gay, eve, bea, dee, cath, ivy, hope};
            abi.Prefs  = new List<Person>() {bob, fred, jon, gav, ian, abe, dan, ed, col, hal};
            bea.Prefs  = new List<Person>() {bob, abe, col, fred, gav, dan, ian, ed, jon, hal};
            cath.Prefs = new List<Person>() {fred, bob, ed, gav, hal, col, ian, abe, dan, jon};
            dee.Prefs  = new List<Person>() {fred, jon, col, abe, ian, hal, gav, dan, bob, ed};
            eve.Prefs  = new List<Person>() {jon, hal, fred, dan, abe, gav, col, ed, ian, bob};
            fay.Prefs  = new List<Person>() {bob, abe, ed, ian, jon, dan, fred, gav, col, hal};
            gay.Prefs  = new List<Person>() {jon, gav, hal, fred, bob, abe, col, ed, dan, ian};
            hope.Prefs = new List<Person>() {gav, jon, bob, abe, ian, dan, hal, ed, col, fred};
            ivy.Prefs  = new List<Person>() {ian, col, hal, gav, fred, bob, abe, ed, jon, dan};
            jan.Prefs  = new List<Person>() {ed, hal, gav, abe, bob, jon, col, ian, fred, dan};
            
            List<Person> men = new List<Person>(abi.Prefs);
            
            int freeMenCount = men.Count;
            while (freeMenCount > 0) {
                foreach (Person guy in men) {
                    if (guy.Fiance == null) {
                        Person gal = guy.NextCandidateNotYetProposedTo();
                        if (gal.Fiance == null) {
                            guy.EngageTo(gal);
                            freeMenCount--;
                        } else if (gal.Prefers(guy)) {
                            guy.EngageTo(gal);
                        }
                    }
                }
            }
            
            foreach (Person guy in men) {
                Console.WriteLine("{0} is engaged to {1}", guy.Name, guy.Fiance.Name);
            }
            Console.WriteLine("Stable = {0}", IsStable(men));
            
            Console.WriteLine("\nSwitching fred & jon's partners");
            Person jonsFiance = jon.Fiance;
            Person fredsFiance = fred.Fiance;
            fred.EngageTo(jonsFiance);
            jon.EngageTo(fredsFiance);
            Console.WriteLine("Stable = {0}", IsStable(men));
        }
        
        public static void Main(string[] args)
        {
            DoMarriage();
        }
    }
}

C++: #include <algorithm>
#include <iostream>
#include <map>
#include <queue>
#include <string>
#include <vector>
using namespace std;

const char *men_data[][11] = {
    { "abe",  "abi","eve","cath","ivy","jan","dee","fay","bea","hope","gay" },
    { "bob",  "cath","hope","abi","dee","eve","fay","bea","jan","ivy","gay" },
    { "col",  "hope","eve","abi","dee","bea","fay","ivy","gay","cath","jan" },
    { "dan",  "ivy","fay","dee","gay","hope","eve","jan","bea","cath","abi" },
    { "ed",   "jan","dee","bea","cath","fay","eve","abi","ivy","hope","gay" },
    { "fred", "bea","abi","dee","gay","eve","ivy","cath","jan","hope","fay" },
    { "gav",  "gay","eve","ivy","bea","cath","abi","dee","hope","jan","fay" },
    { "hal",  "abi","eve","hope","fay","ivy","cath","jan","bea","gay","dee" },
    { "ian",  "hope","cath","dee","gay","bea","abi","fay","ivy","jan","eve" },
    { "jon",  "abi","fay","jan","gay","eve","bea","dee","cath","ivy","hope" }
};
 
const char *women_data[][11] = {
    { "abi",  "bob","fred","jon","gav","ian","abe","dan","ed","col","hal" },
    { "bea",  "bob","abe","col","fred","gav","dan","ian","ed","jon","hal" },
    { "cath", "fred","bob","ed","gav","hal","col","ian","abe","dan","jon" },
    { "dee",  "fred","jon","col","abe","ian","hal","gav","dan","bob","ed" },
    { "eve",  "jon","hal","fred","dan","abe","gav","col","ed","ian","bob" },
    { "fay",  "bob","abe","ed","ian","jon","dan","fred","gav","col","hal" },
    { "gay",  "jon","gav","hal","fred","bob","abe","col","ed","dan","ian" },
    { "hope", "gav","jon","bob","abe","ian","dan","hal","ed","col","fred" },
    { "ivy",  "ian","col","hal","gav","fred","bob","abe","ed","jon","dan" },
    { "jan",  "ed","hal","gav","abe","bob","jon","col","ian","fred","dan" }
};

typedef vector<string> PrefList;
typedef map<string, PrefList> PrefMap;
typedef map<string, string> Couples;


bool prefers(const PrefList &prefer, const string &first, const string &second)
{
    for (PrefList::const_iterator it = prefer.begin(); it != prefer.end(); ++it)
    {
        if (*it == first) return true;
        if (*it == second) return false;
    }
    return false; 
}

void check_stability(const Couples &engaged, const PrefMap &men_pref, const PrefMap &women_pref)
{
    cout << "Stablility:\n";
    bool stable = true;
    for (Couples::const_iterator it = engaged.begin(); it != engaged.end(); ++it)
    {
        const string &bride = it->first;
        const string &groom = it->second;
        const PrefList &preflist = men_pref.at(groom);

        for (PrefList::const_iterator it = preflist.begin(); it != preflist.end(); ++it)
        {
            if (*it == bride) 
                break;
            if (prefers(preflist, *it, bride) && 
                prefers(women_pref.at(*it), groom, engaged.at(*it))) 
            {
                cout << "\t" << *it <<
                    " prefers " << groom <<
                    " over " << engaged.at(*it) <<
                    " and " << groom <<
                    " prefers " << *it <<
                    " over " << bride << "\n";
                stable = false;
            }
        }
    }
    if (stable) cout << "\t(all marriages stable)\n";
}

int main()
{
    PrefMap men_pref, women_pref;
    queue<string> bachelors;

    
    for (int i = 0; i < 10; ++i) 
    {
        for (int j = 1; j < 11; ++j) 
        {
              men_pref[  men_data[i][0]].push_back(  men_data[i][j]);
            women_pref[women_data[i][0]].push_back(women_data[i][j]);
        }
        bachelors.push(men_data[i][0]);
    }

    Couples engaged; 

    cout << "Matchmaking:\n";
    while (!bachelors.empty())
    {
        const string &suitor = bachelors.front();
        const PrefList &preflist = men_pref[suitor];

        for (PrefList::const_iterator it = preflist.begin(); it != preflist.end(); ++it)
        {
            const string &bride = *it;

            if (engaged.find(bride) == engaged.end()) 
            {
                cout << "\t" << bride << " and " << suitor << "\n";
                engaged[bride] = suitor; 
                break;
            }

            const string &groom = engaged[bride];

            if (prefers(women_pref[bride], suitor, groom))
            {
                cout << "\t" << bride << " dumped " << groom << " for " << suitor << "\n";
                bachelors.push(groom); 
                engaged[bride] = suitor; 
                break;
            }
        }
        bachelors.pop(); 
    }

    cout << "Engagements:\n";
    for (Couples::const_iterator it = engaged.begin(); it != engaged.end(); ++it)
    {
        cout << "\t" << it->first << " and " << it->second << "\n";
    }

    check_stability(engaged, men_pref, women_pref);

    cout << "Perturb:\n";
    std::swap(engaged["abi"], engaged["bea"]);
    cout << "\tengage abi with " << engaged["abi"] << " and bea with " << engaged["bea"] << "\n";

    check_stability(engaged, men_pref, women_pref);
}