Task 68. UniLecs. Count of n digit numbers using only 4 and 7, where cannot be 3 same digits in a row.

twoDigits.js

/*
 *  It's not technically correct as there are possible numbers
 *  that have more than one subsequence of exact digits: 7774777
 *  but here it's considered only one possible subsequence of exact digits
 */
const answer = n => {
  const allPossibleCount = n === 0 ? 0 : Math.pow(2, n);
  const threeOrMoreSameDigitInARowCount = Array(Math.max(n - (3 + index) + 1, 0))
    .fill(0)
    .map((_, index) => (n - 3 + index) * 2)
    .reduce((a, b) => a + b, 0);
  return allPossibleCount - threeOrMoreSameDigitInARowCount;
};

for (let i = 0; i <= 30; i++) {
  console.log(answer(i));
}

// 0
// 2
// 4
// 6
// 10
// 20
// 44
// 98
// 214
// 456
// 952
// 1958
// 3986
// 8060
// 16228
// 32586
// 65326
// 130832
// 261872
// 523982
// 1048234
// 2096772
// 4193884
// 8388146
// 16776710
// 33553880
// 67108264
// 134217078
// 268434754
// 536870156
// 1073741012