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Introduction

LeetCode 273: Integer to English Words is a classic string formatting and recursion problem. It asks you to convert an integer into its spoken English equivalent. Think of it like building your own version of num2words().

This is a popular system design building block, commonly used in:

• Cheque writing software
• Accounting tools
• Assistive tech (text-to-speech)
• Natural Language Processing systems

Problem Statement

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

You must return the result as a string without extra spaces.

Examples

python

Input: num = 123
Output: "One Hundred Twenty Three"

Input: num = 12345
Output: "Twelve Thousand Three Hundred Forty Five"

Input: num = 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

🧠 Strategy & Breakdown

We break the number into chunks of three digits (hundreds, thousands, millions, billions), and convert each using recursion or mapping.

🔢 Step 1: Define Core Words

We need three groups of mappings:

python

<20       = ['One', 'Two', ..., 'Nineteen']
tens      = ['Twenty', 'Thirty', ..., 'Ninety']
thousands = ['', 'Thousand', 'Million', 'Billion']

🔁 Step 2: Convert 3-Digit Chunk

Create a helper function to convert a number less than 1000:

• For example:
• 123 → "One Hundred Twenty Three"
• 508 → "Five Hundred Eight"

🔁 Step 3: Process Chunks From Right to Left

• For 1,234,567:
• 567 → "Five Hundred Sixty Seven"
• 234 → "Two Hundred Thirty Four Thousand"
• 1 → "One Million"

Join them together in reverse order.

✅ Python Code

python

class Solution:
    def numberToWords(self, num: int) -> str:
        if num == 0:
            return "Zero"
        
        below_20 = [
            "", "One", "Two", "Three", "Four", "Five", "Six",
            "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve",
            "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen",
            "Eighteen", "Nineteen"
        ]
        
        tens = [
            "", "", "Twenty", "Thirty", "Forty", "Fifty",
            "Sixty", "Seventy", "Eighty", "Ninety"
        ]
        
        thousands = ["", "Thousand", "Million", "Billion"]
        
        def helper(n):
            if n == 0:
                return ""
            elif n < 20:
                return below_20[n] + " "
            elif n < 100:
                return tens[n // 10] + " " + helper(n % 10)
            else:
                return below_20[n // 100] + " Hundred " + helper(n % 100)
        
        res = ""
        for i in range(len(thousands)):
            if num % 1000 != 0:
                res = helper(num % 1000) + thousands[i] + " " + res
            num //= 1000
        
        return res.strip()

🧪 Dry Run

Input: num = 1234567

Chunks:

• 567 → "Five Hundred Sixty Seven"
• 234 → "Two Hundred Thirty Four Thousand"
• 1 → "One Million"

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

⏱️ Time and Space Complexity

• Time Complexity: O(log n)
• (Each digit is processed once.)
• Space Complexity: O(1) auxiliary (ignoring output string)

✅ Edge Cases

• 0 → "Zero"
• 1000000 → "One Million"
• Leading zeros are ignored
• Extra spaces are cleaned using .strip()

📌 Notes

• You can add Billion safely as input is guaranteed to be ≤ 2³¹ - 1
• This is a great example of modular problem solving:
• Break number into chunks
• Handle small parts
• Compose the answer

✅ Conclusion

LeetCode 273: Integer to English Words is a practical, real-world problem teaching:

• String decomposition
• Recursion and modular logic
• Human-readable formatting