Einstein Problem - http://www.iflscience.com/editors-blog/solving-einsteins-riddle

einstein.js

const startTime = Date.now();
function generatePermutations(dictionary) {
  const results = [];
  const permute = (array, position) => {
    if (position === array.length - 1) {
      results.push(array);
    } else {
      for (let i = position; i < array.length; i++) {
        const swap = array[position];
        array.splice(position, 1, array[i]);
        array.splice(i, 1, swap);
        permute(array.slice(), (position + 1));
      }
    }
  };
  permute(dictionary, 0);
  return results;
};

const colors = generatePermutations(['red', 'white', 'green', 'yellow', 'blue']);
const drinks = generatePermutations(['tea', 'coffee', 'milk', 'beer', 'water']);
const smokes = generatePermutations(['Pall Mall', 'Dunhill', 'Blends', 'Bluemasters', 'Prince']);
const pets = generatePermutations(['dogs', 'birds', 'cats', 'fish', 'a horse']);
const nationalities = generatePermutations(['Brit', 'Swede', 'Dane', 'Norwegian', 'German']);

function print(color, drink, smoke, pet, nationality) {
  console.log('------');
  [0, 1, 2, 3, 4].forEach(index => {
    console.log('House %s: The %s lives in a %s house, drinks %s, smokes %s, and has %s.',
      index + 1, nationality[index], color[index], drink[index], smoke[index], pet[index]);
  })
  console.log('------');
}

const reasons = {};


let n = 0;
function inc(reason) {
  reasons[reason] = (reasons[reason] || 0) + 1;
}


nationalities.forEach(nationality => {
  // 9. The Norwegian lives in the first house.
  if (nationality[0] !== 'Norwegian') {
    return inc('9. The Norwegian lives in the first house.');
  }
  smokes.forEach(smoke => {
    pets.forEach(pet => {
      // 11. The owner who keeps the horse lives next to the one who smokes Dunhill.
      if (smoke[pet.indexOf('a horse') - 1] !== 'Dunhill' && smoke[pet.indexOf('a horse') + 1] !== 'Dunhill') {
        return inc('11. The owner who keeps the horse lives next to the one who smokes Dunhill.');
      }
      // 2. The Swede keeps dogs as pets.
      if (pet[nationality.indexOf('Swede')] !== 'dogs') {
        return inc('2. The Swede keeps dogs as pets.');
      }
      // 10. The owner who smokes Blends lives next to the one who keeps cats.
      if (pet[smoke.indexOf('Blends') - 1] !== 'cats' && pet[smoke.indexOf('Blends') + 1] !== 'cats') {
        return inc('10. The owner who smokes Blends lives next to the one who keeps cats.');
      }
      // 6. The owner who smokes Pall Mall rears birds.
      if (pet[smoke.indexOf('Pall Mall')] !== 'birds') {
        return inc('6. The owner who smokes Pall Mall rears birds.');
      }
      drinks.forEach(drink => {
        // 8. The owner living in the center house drinks milk.
        if (drink[2] !== 'milk') {
          return inc('8. The owner living in the center house drinks milk.');
        }
        colors.forEach(color => {
          // 1. The Brit lives in the red house.
          if (color[nationality.indexOf('Brit')] !== 'red') {
            return inc('1. The Brit lives in the red house.');
          }
          // 3. The Dane drinks tea.
          if (drink[nationality.indexOf('Dane')] !== 'tea') {
            return inc('3. The Dane drinks tea.');
          }
          // 4. The green house is on the immediate left of the white house.
          if (color[color.indexOf('green') + 1] !== 'white') {
            return inc('4. The green house is on the immediate left of the white house.');
          }
          // 5. The green house's owner drinks coffee.
          if (drink[color.indexOf('green')] !== 'coffee') {
            return inc('5. The green house\'s owner drinks coffee.');
          }
          // 7. The owner of the yellow house smokes Dunhill.
          if (smoke[color.indexOf('yellow')] !== 'Dunhill') {
            return inc('7. The owner of the yellow house smokes Dunhill.');
          }
          // 12. The owner who smokes Bluemasters drinks beer.
          if (drink[smoke.indexOf('Bluemasters')] !== 'beer') {
            return inc('12. The owner who smokes Bluemasters drinks beer.');
          }
          // 13. The German smokes Prince.
          if (smoke[nationality.indexOf('German')] !== 'Prince') {
            return inc('13. The German smokes Prince.');
          }
          // 14. The Norwegian lives next to the blue house.
          if (color[nationality.indexOf('Norwegian') - 1] !== 'blue' &&
            color[nationality.indexOf('Norwegian') + 1] !== 'blue') {
            return inc('14. The Norwegian lives next to the blue house.');
          }
          // 15. The owner who smokes Blends lives next to the one who drinks water.
          if (drink[smoke.indexOf('Blends') - 1] !== 'water' &&
            drink[smoke.indexOf('Blends') + 1] !== 'water') {
            return inc('15. The owner who smokes Blends lives next to the one who drinks water.');
          }

          print(color, drink, smoke, pet, nationality);
        });
      });
    });
  });
});

// Print reasons
let total = 0;
const totalTime = Date.now() - startTime;
Object
  .keys(reasons)
  .sort((a, b) => reasons[b] - reasons[a]) // reverse order (highest first)
  .forEach(reason => {
    total += reasons[reason];
    console.log(reasons[reason], reason);
  });
console.log('--------\n%d total nodes tested. (%d%% of total space)  %s seconds', total, (100 * total / Math.pow(120, 5)).toPrecision(3), totalTime / 1000);

/****
 OUTPUT:
 ------
 House 1: The Norwegian lives in a yellow house, drinks water, smokes Dunhill, and has cats.
 House 2: The Dane lives in a blue house, drinks tea, smokes Blends, and has a horse.
 House 3: The Brit lives in a red house, drinks milk, smokes Pall Mall, and has birds.
 House 4: The German lives in a green house, drinks coffee, smokes Prince, and has fish.
 House 5: The Swede lives in a white house, drinks beer, smokes Bluemasters, and has dogs.
 ------
 3096576 '1. The Brit lives in the red house.'
 631296 '3. The Dane drinks tea.'
 235008 '11. The owner who keeps the horse lives next to the one who smokes Dunhill.'
 129024 '8. The owner living in the center house drinks milk.'
 116544 '4. The green house is on the immediate left of the white house.'
 88992 '2. The Swede keeps dogs as pets.'
 21024 '5. The green house\'s owner drinks coffee.'
 14400 '10. The owner who smokes Blends lives next to the one who keeps cats.'
 5856 '6. The owner who smokes Pall Mall rears birds.'
 4212 '7. The owner of the yellow house smokes Dunhill.'
 848 '12. The owner who smokes Bluemasters drinks beer.'
 179 '13. The German smokes Prince.'
 96 '9. The Norwegian lives in the first house.'
 40 '14. The Norwegian lives next to the blue house.'
 --------
 4344095 total nodes tested. (0.0175% of total space)  1.675 seconds
 ****/