08. Integer to English Words

Difficulty:HardPattern:3-Digit Chunk ParsingTime:O(1)Space:O(1)

Convert a non-negative integer num to its English words representation.

How It Works & Algorithmic Design

Solves using Chunking by Thousands with Static Word Arrays:

Static Lookup Arrays: Define arrays for units ["", "One", ...], teens ["Ten", "Eleven", ...], tens ["", "", "Twenty", ...], and thousands ["", "Thousand", "Million", "Billion"].
Three-Digit Helper helper(n):
- If $n < 20$, lookup units/teens.
- If $n < 100$, return tens[n/10] + helper(n%10).
- If $n \ge 100$, return units[n/100] + " Hundred " + helper(n%100).
Loop Thousands: Process num in 3-digit chunks, appending thousand group names.

Why This Approach Works Best

Recursively converts 3-digit groups in $O(1)$ constant time.

Example Walkthrough

Input: num = 1234567

Target integer.

Chunks: 1 (Million) + 234 (Thousand) + 567

3-digit groups.

Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

Converted text.

Answer: Integer to English Words implementation

function numberToWords(num) {
  if (num === 0) return 'Zero';
  const LESS_THAN_20 = ['', 'One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Eleven', 'Twelve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen', 'Seventeen', 'Eighteen', 'Nineteen'];
  const TENS = ['', '', 'Twenty', 'Thirty', 'Forty', 'Fifty', 'Sixty', 'Seventy', 'Eighty', 'Ninety'];
  const THOUSANDS = ['', 'Thousand', 'Million', 'Billion'];
  function helper(n) {
    if (n === 0) return '';
    else if (n < 20) return LESS_THAN_20[n] + ' ';
    else if (n < 100) return TENS[Math.floor(n / 10)] + ' ' + helper(n % 10);
    else return LESS_THAN_20[Math.floor(n / 100)] + ' Hundred ' + helper(n % 100);
  }
  let res = '', i = 0;
  while (num > 0) {
    if (num % 1000 !== 0) {
      res = helper(num % 1000) + THOUSANDS[i] + ' ' + res;
    }
    num = Math.floor(num / 1000);
    i++;
  }
  return res.trim().replace(/\s+/g, ' ');
}

💡 Parse number in groups of 3 digits appending Thousand, Million, and Billion suffix scale tags.

Key Takeaways

Chunks integers into 3-digit groups (thousands scale).
Helper function handles standard sub-1000 numbers recursively.
Executes in $O(1)$ bounded time.