const alphabet = "abcdefghijklmnopqrstuvwxyz";

const start = 'X' + alphabet.repeat(100);
const middle = alphabet.repeat(50) + 'X' + alphabet.repeat(50);
const end = alphabet.repeat(100) + 'X';

function firstNonRepeatingChar(str) {
  for (let i = 0; i < str.length; i++) {
    if (str.indexOf(str[i]) === str.lastIndexOf(str[i])) {
      return str[i]
    }
  }

  return null
}

function firstNonRepeatingCharV2(str) {
  const seen = new Set()

  for (let i = 0; i < str.length; i++) {
    const char = str[i]

    if (seen.has(char)) {
      continue
    }

    if (i === str.lastIndexOf(char)) {
      return char
    }

    seen.add(char)
  }

  return null
}