Topsy Turvy Numbers

This was an interesting problem, it requires the filtering of numbers that do not look the same upside down as they do right-side up. Also, you are then required to find the palindromes within the given range.

Topsy turvy numbers are numbers that when inverted (rotated 180 degrees) are unchanged. The most recent topsy turvy years were 1961 and 1881. The next one won’t be until 6009. Mathemeticians have shown little interest in these numbers, but Edabitians are always up for a challenge:

Write a function that accepts two non-negative integers and returns an array of all topsy turvy numbers inclusively in that range. Examples

topsyTurvy(0, 10) ➞ [0, 1, 8]

topsyTurvy(10, 100) ➞ [11, 69, 88, 96]

topsyTurvy(1000, 2000) ➞ [1001, 1111, 1691, 1881, 1961]

My solution

Notice how I used the Array.from method to make the range of numbers nums between the lo and hi numbers.

function topsyTurvy(lo, hi) {
	const nums = Array.from({ length: (hi - lo)  + 1}, (_, i) => lo + i );
  function tt(num) {
    const rev = +[...String(num)].reverse().map(x=> x === '6' ? '9' : x === '9' ? '6' : x.match(/[23457]/g) ? '' : x).join('')
    return rev === num ? num : ''
  }
  return nums.map(x=> tt(x)).filter(String)
}

topsyTurvy(0, 10) //  [0, 1, 8]
topsyTurvy(10, 100) //  [11, 69, 88, 96]
topsyTurvy(1000, 2000) //  [1001, 1111, 1691, 1881, 1961]

Other solutions

const ud = [0, 1, , , , , 9, , 8, 6];
const flip = n => [...''+n].reverse().map(v => ud[v]).join('');
const isTT = n => (''+n) === flip(n);

const topsyTurvy = (l, h) => [...Array(h - l + 1)].map((_,i) => i + l).filter(isTT);
// mbbentley

const reverseNumbers = {
  0: 0,
  1: 1,
  6: 9,
  8: 8,
  9: 6,
};

const isTopsyTurvy = num => {
  const str = String(num);
  const rotated = Array.from(str, digit => reverseNumbers[digit])
    .reverse()
    .join('');

  return str === rotated;
};

const topsyTurvy = (min, max) => {
  const results = [];

  for (let num = min; num <= max; num++) {
    if (isTopsyTurvy(num)) results.push(num);
  }

  return results;
};
// Pustur