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const alphanumericStr = 'ab4c13!!?!@#$%^&567'

const digits = []  // [{ letter: 4, position: 2 }]
const result = [] // [a, b, undefined, c]

for (let i = 0; i < alphanumericStr.length; i += 1) {
  const letter = alphanumericStr[i]

  if (isFinite(letter)) {
    digits.push({
      letter: letter,
      position: i
    })
  } else {
    result[i] = letter
  }
}

const reversedDigits = digits.slice()
  
reversedDigits.reverse()

for (let i = 0; i < digits.length; i += 1) {
  const { letter } = digits[i]
  const { position } = reversedDigits[i]

  result[position] = letter
}

console.log(result.join(''))