Integer to English Words

hardWeak areas: redo your mistakes notebook + new82 stepsJava · Python · C++

Spell out a non-negative integer in English. Split it into groups of three digits (ones, thousands, millions, billions). Each group under 1000 is spelled with a small helper that handles hundreds, the teens, and tens; then append the group's scale word.

▶ Watch the animated trace

Traced input: num = 1234567 → "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven". Original problem: leetcode.com/problems/integer-to-english-words/

Complexity

BestO(1)

AverageO(1)

WorstO(1)

SpaceO(1)

At most four groups for a 32-bit integer.

Key idea

English repeats the same pattern for every three digits, plus a scale word.
Numbers below 20 are irregular, so look them up directly.
Skip groups that are 0 (no "Zero Thousand").
Zero itself is a special case.

Common mistakes

Printing "Zero" inside larger numbers.
Extra or trailing spaces; trim at the end.
Teens like 13 are not "Ten Three".

Solution

Java

static final String[] BELOW_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
static final String[] TENS = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
static final String[] SCALES = {"", "Thousand", "Million", "Billion"};

String numberToWords(int num) {
    if (num == 0) return "Zero";
    String res = "";
    int scale = 0;
    while (num > 0) {
        int chunk = num % 1000;
        if (chunk != 0) res = three(chunk) + SCALES[scale] + " " + res;
        num /= 1000;
        scale++;
    }
    return res.trim();
}

String three(int n) {
    String s = "";
    if (n >= 100) {
        s += BELOW_20[n / 100] + " Hundred ";
        n %= 100;
    }
    if (n >= 20) {
        s += TENS[n / 10] + " ";
        n %= 10;
    }
    if (n > 0) s += BELOW_20[n] + " ";
    return s;
}

Python

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"]
SCALES = ["", "Thousand", "Million", "Billion"]


def numberToWords(num: int) -> str:
    if num == 0: return "Zero"
    res, scale = "", 0
    while num > 0:
        chunk = num % 1000
        if chunk: res = three(chunk) + SCALES[scale] + " " + res
        num //= 1000
        scale += 1
    return " ".join(res.split())


def three(n: int) -> str:
    s = ""
    if n >= 100:
        s += BELOW_20[n // 100] + " Hundred "
        n %= 100
    if n >= 20:
        s += TENS[n // 10] + " "
        n %= 10
    if n > 0: s += BELOW_20[n] + " "
    return s

C++

const vector<string> BELOW_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
const vector<string> TENS = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
const vector<string> SCALES = {"", "Thousand", "Million", "Billion"};

string three(int n) {
    string s;
    if (n >= 100) {
        s += BELOW_20[n / 100] + " Hundred ";
        n %= 100;
    }
    if (n >= 20) {
        s += TENS[n / 10] + " ";
        n %= 10;
    }
    if (n > 0) s += BELOW_20[n] + " ";
    return s;
}

string numberToWords(int num) {
    if (num == 0) return "Zero";
    string res;
    int scale = 0;
    while (num > 0) {
        int chunk = num % 1000;
        if (chunk != 0) res = three(chunk) + SCALES[scale] + " " + res;
        num /= 1000;
        scale++;
    }
    // collapse the extra spaces left by empty scale words
    string out;
    istringstream in(res);
    for (string w; in >> w;) out += (out.empty() ? "" : " ") + w;
    return out;
}

Edge cases

Each one has its own trace in the interactive view.

Case	Input	Answer
Zero Special case.	`num = 0`	`"Zero"`
Teens and empty groups The thousands group is 0 and is skipped.	`num = 1000013`	`"One Million Thirteen"`
Largest int All four groups are used.	`num = 2147483647`	`"Two Billion One Hundred Forty Seven Million Four Hundred Eighty Three Thousand Six Hundred Forty Seven"`
Round hundred No trailing words.	`num = 100`	`"One Hundred"`