Alternate vowel and consonant string

<script>

// JavaScript implementation of alternate vowel and
// consonant string

// 'ch' is vowel or not
function isVowel(ch)
{
    if (ch == 'a' || ch == 'e' || ch == 'i' ||
            ch == 'o' || ch =='u')
        return true;
    return false;
}

// create alternate vowel and consonant string
// str1[0...l1-1] and str2[start...l2-1]
function createAltStr(str1, str2,start,l)
{
    let finalStr = "";

    // first adding character of vowel/consonant
    // then adding character of consonant/vowel
    for (let i=0, j=start; j<l; i++, j++)
        finalStr = (finalStr + str1[i] + str2[j]);
    return finalStr;
}

// function to find the required
// alternate vowel and consonant string
function findAltStr(str)
{
    let nv = 0, nc = 0;
    let vstr = "", cstr = "";
    let l = str.length;
    for (let i=0; i<l; i++)
    {
        let ch = str[i];

        // count vowels and update vowel string
        if (isVowel(ch))
        {
            nv++;
            vstr = vstr + ch;
        }

        // count consonants and update consonant
        // string
        else
        {
            nc++;
            cstr = cstr + ch;
        }
    }

    // no such string can be formed
    if (Math.abs(nv-nc) >= 2)
        return "no such string";

    // remove first character of vowel string
    // then create alternate string with
    // cstr[0...nc-1] and vstr[1...nv-1]
    if (nv > nc)
        return (vstr[0] + createAltStr(cstr, vstr, 1, nv));

    // remove first character of consonant string
    // then create alternate string with
    // vstr[0...nv-1] and cstr[1...nc-1]
    if (nc > nv)
        return (cstr[0] + createAltStr(vstr, cstr, 1, nc));

    // if both vowel and consonant
    // strings are of equal length
    // start creating string with consonant
    if (cstr.at(0) < vstr.at(0))
        return createAltStr(cstr, vstr, 0, nv);

    // start creating string with vowel
    return createAltStr(vstr, cstr, 0, nc);
}

// Driver program to test above
let str = "geeks";
document.write(findAltStr(str));

// This code is contributed by Shinjan_Patra
</script>

function findAltStr(s) {
  const n = s.length;
  const mp1 = new Array(26).fill(0);
  const mp2 = new Array(26).fill(0); // to store vowels and consonants
  let v = 0;
  let c = 0;
  for (const ch of s) {
    if (['a', 'e', 'i', 'o', 'u'].includes(ch)) { // if it's vowel
      mp1[ch.charCodeAt(0) - 'a'.charCodeAt(0)] += 1;
      v += 1;
    } else { // consonant
      mp2[ch.charCodeAt(0) - 'a'.charCodeAt(0)] += 1;
      c += 1;
    }
  }

  if (Math.abs(v - c) > 1) {
    return "no such string"; // if their diff is greater than one
    // then string with alternate vowel and
    // consonant cannot be made
  }

  let it1 = 0;
  let it2 = 0;
  let i = 0;
  while (it1 < mp1.length && mp1[it1] == 0) {
    it1 += 1; // to find first vowel
  }
  while (it2 < mp2.length && mp2[it2] == 0) {
    it2 += 1; // to find first consonant
  }

  let f = c > v; // if number of consonant is greater then
  // we will place consonant first else vowel
  if (v === c) {
    f = it1 > it2; // if both are equal then check which
    // is lexiographically smaller
  }

  const new_str = new Array(n);
  while (it1 < mp1.length && it2 < mp2.length && i < n) {
    if (f) {
      new_str[i] = String.fromCharCode(it2 + 'a'.charCodeAt(0));
      mp2[it2] -= 1;
      while (it2 < mp2.length && mp2[it2] === 0) {
        it2 += 1;
      }
      f = false; // this will trigger to place vowel
      // next
    } else {
      new_str[i] = String.fromCharCode(it1 + 'a'.charCodeAt(0));
      mp1[it1] -= 1;
      while (it1 < mp1.length && mp1[it1] === 0) {
        it1 += 1;
      }
      f = true; // this will trigger to place
      // consonant next
    }
    i += 1;
  }

  if (it1 !== mp1.length) {
    new_str[i] = String.fromCharCode(it1 + 'a'.charCodeAt(0)); // if one vowel left
  } else if (it2 !== mp2.length) {
    new_str[i] = String.fromCharCode(it2 + 'a'.charCodeAt(0)); // if one consonant left
  }

  return new_str.join('');
}

// Driver program to test above
const str = "geeks";
console.log(findAltStr(str));