DouKaku?

#include <iostream>
#include <iomanip>
#include <string>
#include <vector>
#include <algorithm>
#include <cstdlib> // rand, RAND_MAX
#include <cassert>
#include <ctime>

const std::string goal = "METHINKS IT IS LIKE A WEASEL";

size_t random(size_t n) // [0, n)
{
    assert(n > 0);

    return static_cast<size_t>(std::rand() * (n + 1.0) / (RAND_MAX + 1.0));
}

char random_char()
{
    static const char table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ ";

    return table[random(sizeof(table))];
}

std::string random_string()
{
    std::string s;

    s.resize(goal.size());

    std::generate(s.begin(), s.end(), random_char);

    return s;
}

class sentence
{
public:
    explicit sentence(const std::string& s = random_string());

    const std::string& str() const
    {
        return _s;
    }

    friend bool operator<(const sentence& lhs, const sentence& rhs)
    {
        return lhs._diff < rhs._diff;
    }

private:
    std::string _s;
    size_t _diff;
};

sentence::sentence(const std::string& s) : _s(s), _diff(0)
{
    assert(s.size() == goal.size());

    for (size_t i = 0; i < s.size(); ++i)
    {
        if (s[i] != goal[i])
        {
            ++_diff;
        }
    }
}

int main()
{
    std::srand(std::time(NULL));

    const size_t count = 300;

    std::vector<sentence> v(count);

    for (size_t generation = 1; ; ++generation)
    {
        std::sort(v.begin(), v.end());

        v.erase(v.begin() + count, v.end());

        std::cout << std::setw(4) << generation << ": " << v.front().str() << std::endl;

        if (v.front().str() == goal)
        {
            break;
        }

        for (size_t i = 0; i < count; ++i)
        {
            std::string s = v[i].str();

            s[random(s.size())] = random_char();

            v.push_back(sentence(s));
        }
    }
}