273. Integer to English Words

2026, Aug 27 2 mins read

1. Description

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

2. Example

Example 1

Input: num = 123
Output: “One Hundred Twenty Three”

Example 2

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

Example 3

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

3. Constraints

0 <= num <= 2$^{31}$ - 1

4. Solutions

Greedy

n = num
Time complexity: O(n)
Space complexity: O(1)

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class Solution {
public:
    string numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }

        string result;
        int index = 0;

        while (num > 0) {
            int current = num % 1000;

            if (current != 0) {
                string part = convert_to_words(current);

                if (!units[index].empty()) {
                    part += " " + units[index];
                }

                if (!result.empty()) {
                    part += " ";
                }

                result = part + result;
            }

            ++index;
            num /= 1000;
        }

        return result;
    }

private:
    const array<string, 10>
        ones{"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};

    const array<string, 10> teens{
        "Ten",
        "Eleven",
        "Twelve",
        "Thirteen",
        "Fourteen",
        "Fifteen",
        "Sixteen",
        "Seventeen",
        "Eighteen",
        "Nineteen"};

    const array<string, 10>
        tens{"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};

    const array<string, 4> units{"", "Thousand", "Million", "Billion"};

    string convert_to_words(int num) {
        string result;

        if (num >= 100) {
            result += ones[num / 100] + " Hundred";
            num %= 100;

            if (num > 0) {
                result += " ";
            }
        }

        if (num >= 20) {
            result += tens[num / 10];

            if (num % 10 > 0) {
                result += " " + ones[num % 10];
            }
        } else if (num >= 10) {
            result += teens[num - 10];
        } else if (num > 0) {
            result += ones[num];
        }

        return result;
    }
};

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