Alternating characters

alternating-characters.js

// Time complexity: O(n) where n is the number of characters.
// Space complexity: O(1) for a few variables.
function alternatingCharacters(x) {
  const len = x.length;

  if (len < 2) return 0;

  let lastChar = x[0];
  let count = 0;
  let countA = lastChar === 'a' ? 1 : 0;
  let countB = lastChar === 'b' ? 1 : 0;
  for (let i = 1; i < len; i += 1) {
    const n = x[i];

    // If current letter is the same as the last seen letter,
    // increment `count`.
    if (lastChar === n) count += 1;
    if ('a' === n) countA += 1;
    if ('b' === n) countB += 1;

    lastChar = n;
  }

  // Here compare for cases where `count` is 0 as all letters
  // are alternating but the number of A and the number of B
  // are not the same.
  return count === 0 ?
    Math.abs(countA - countB) :
    count;
}

function main() {
  const a = [
    'aaaa',
    'bbbbb',
    'abababab',
    'bababa',
    'aaabbb',

    'aaaab',
    'bbbbba',
    'ababababa',
    'bababab',
  ];

  a.forEach((n) => {
    console.log('Alternating characters: ', n, alternatingCharacters(n));
  });
}

console.clear();
main();