Integer to English Words

Problem#

Convert a non-negative integer to its English-word representation.

Examples#

123 → "One Hundred Twenty Three".
12345 → "Twelve Thousand Three Hundred Forty Five".

Constraints#

0 <= num <= 2^31 - 1.

Approach#

Split the number into groups of three digits (thousands). For each group emit “hundreds tens units” via lookup tables. Append the magnitude suffix per group.

Solution#

class Solution {
    vector<string> below20 = {"","One","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Eleven","Twelve","Thirteen","Fourteen","Fifteen","Sixteen","Seventeen","Eighteen","Nineteen"};
    vector<string> tens = {"","","Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety"};
    string under1000(int n) {
        string s;
        if (n >= 100) { s += below20[n / 100] + " Hundred"; n %= 100; if (n) s += ' '; }
        if (n >= 20) { s += tens[n / 10]; n %= 10; if (n) s += ' '; }
        if (n > 0 && n < 20) s += below20[n];
        return s;
    }
public:
    string numberToWords(int num) {
        if (num == 0) return "Zero";
        vector<string> suffix = {"", " Thousand", " Million", " Billion"};
        string out;
        int idx = 0;
        while (num > 0) {
            int part = num % 1000;
            if (part) {
                string chunk = under1000(part) + suffix[idx];
                out = chunk + (out.empty() ? "" : " ") + out;
            }
            num /= 1000;
            ++idx;
        }
        return out;
    }
};

func numberToWords(num int) string {
    if num == 0 {
        return "Zero"
    }
    below20 := []string{"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"}
    tens := []string{"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"}
    suffix := []string{"", " Thousand", " Million", " Billion"}
    under1000 := func(n int) string {
        s := ""
        if n >= 100 {
            s += below20[n/100] + " Hundred"
            n %= 100
            if n > 0 {
                s += " "
            }
        }
        if n >= 20 {
            s += tens[n/10]
            n %= 10
            if n > 0 {
                s += " "
            }
        }
        if n > 0 && n < 20 {
            s += below20[n]
        }
        return s
    }
    out := ""
    idx := 0
    for num > 0 {
        part := num % 1000
        if part > 0 {
            chunk := under1000(part) + suffix[idx]
            if out == "" {
                out = chunk
            } else {
                out = chunk + " " + out
            }
        }
        num /= 1000
        idx++
    }
    return out
}

class Solution:
    def numberToWords(self, num: int) -> str:
        if num == 0:
            return "Zero"
        below20 = ["", "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"]
        suffix = ["", " Thousand", " Million", " Billion"]

        def under1000(n: int) -> str:
            s = ""
            if n >= 100:
                s += below20[n // 100] + " Hundred"
                n %= 100
                if n > 0:
                    s += " "
            if n >= 20:
                s += tens[n // 10]
                n %= 10
                if n > 0:
                    s += " "
            if 0 < n < 20:
                s += below20[n]
            return s

        out = ""
        idx = 0
        while num > 0:
            part = num % 1000
            if part > 0:
                chunk = under1000(part) + suffix[idx]
                out = chunk if not out else chunk + " " + out
            num //= 1000
            idx += 1
        return out

function numberToWords(num) {
    if (num === 0) return "Zero";
    const below20 = ["", "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 suffix = ["", " Thousand", " Million", " Billion"];

    function under1000(n) {
        let s = "";
        if (n >= 100) {
            s += below20[Math.floor(n / 100)] + " Hundred";
            n %= 100;
            if (n > 0) s += " ";
        }
        if (n >= 20) {
            s += tens[Math.floor(n / 10)];
            n %= 10;
            if (n > 0) s += " ";
        }
        if (n > 0 && n < 20) {
            s += below20[n];
        }
        return s;
    }

    let out = "";
    let idx = 0;
    while (num > 0) {
        const part = num % 1000;
        if (part > 0) {
            const chunk = under1000(part) + suffix[idx];
            out = out === "" ? chunk : chunk + " " + out;
        }
        num = Math.floor(num / 1000);
        idx++;
    }
    return out;
}

class Solution {
    private static final String[] BELOW20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine",
        "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private static final String[] TENS = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    private static final String[] SUFFIX = {"", " Thousand", " Million", " Billion"};

    private String under1000(int n) {
        StringBuilder s = new StringBuilder();
        if (n >= 100) {
            s.append(BELOW20[n / 100]).append(" Hundred");
            n %= 100;
            if (n > 0) s.append(" ");
        }
        if (n >= 20) {
            s.append(TENS[n / 10]);
            n %= 10;
            if (n > 0) s.append(" ");
        }
        if (n > 0 && n < 20) {
            s.append(BELOW20[n]);
        }
        return s.toString();
    }

    public String numberToWords(int num) {
        if (num == 0) return "Zero";
        String out = "";
        int idx = 0;
        while (num > 0) {
            int part = num % 1000;
            if (part > 0) {
                String chunk = under1000(part) + SUFFIX[idx];
                out = out.isEmpty() ? chunk : chunk + " " + out;
            }
            num /= 1000;
            idx++;
        }
        return out;
    }
}

function numberToWords(num: number): string {
    if (num === 0) return "Zero";
    const below20: string[] = ["", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine",
        "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen",
        "Seventeen", "Eighteen", "Nineteen"];
    const tens: string[] = ["", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"];
    const suffix: string[] = ["", " Thousand", " Million", " Billion"];

    const under1000 = (n: number): string => {
        let s = "";
        if (n >= 100) {
            s += below20[Math.floor(n / 100)] + " Hundred";
            n %= 100;
            if (n > 0) s += " ";
        }
        if (n >= 20) {
            s += tens[Math.floor(n / 10)];
            n %= 10;
            if (n > 0) s += " ";
        }
        if (n > 0 && n < 20) {
            s += below20[n];
        }
        return s;
    };

    let out = "";
    let idx = 0;
    while (num > 0) {
        const part = num % 1000;
        if (part > 0) {
            const chunk = under1000(part) + suffix[idx];
            out = out === "" ? chunk : chunk + " " + out;
        }
        num = Math.floor(num / 1000);
        idx++;
    }
    return out;
}

Editorial#

The trick is keeping spaces clean. Build each thousand-group’s chunk, then prepend it to the accumulator with a single space separator only when both sides are non-empty. The num == 0 corner case must be special-cased — every other digit would produce an empty string.

Complexity#

Time: O(log n).
Space: O(log n) for the output.

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