273. 整数转换英文表示

题目描述

输入一个非负整数 num，把它转写成英文读法并以字符串返回。

按英文习惯，每三位一组，分别带上 Thousand、Million、Billion 等单位；每组内用 Hundred 以及 Twenty、Thirty 之类的十位词和 One 到 Nineteen 的基础词表示。为零的部分不读出来，num = 0 时返回 "Zero"。

每个单词首字母大写，单词之间只用一个空格隔开，开头和结尾都不能有空格。

示例 1

输入：num = 123
输出："One Hundred Twenty Three"

解释：123 读作一百二十三。

示例 2

输入：num = 12345
输出："Twelve Thousand Three Hundred Forty Five"

解释：12345 拆成 12 千和 345。

示例 3

输入：num = 1234567
输出："One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

解释：1234567 拆成 1 百万、234 千和 567。

数据范围

0 <= num <= 2^31 - 1

解题思路

英文读数是三位一组：从高到低依次是 Billion、Million、Thousand 和最后三位。对每一组 part = num // 1000^i % 1000，若为 0 就整组跳过；否则按“百位 → 20 及以上的整十 → 1–19”三段拼出单词，再跟上这一组的单位。所有单词放进列表，最后用一个空格连接，天然不会有多余空格。

为什么正确

处理完第 i 组后，res 是 num 中高于 1000^i 的部分的英文读法。

复杂度分析

时间 O(1)：最多 4 组，每组至多 4 个单词；空间 O(1)。

易错点

容易出错的地方有三个：num = 0 要单独返回 "Zero"；10–19 是不规则单词，不能拆成“十位 + 个位”；像 1000000 这样的数，整组为 0 时不能输出单位（不能出现 "One Million Thousand"）。

参考代码

Python、Java、C++、Go、JavaScript、TypeScript 六种语言，逻辑一致。

class Solution:
    def numberToWords(self, num: int) -> str:
        if num == 0:
            return 'Zero'
        ones = ['', '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']
        units = ['', 'Thousand', 'Million', 'Billion']
        res = []
        for i in range(3, -1, -1):
            part = num // 1000 ** i % 1000
            if part == 0:
                continue
            words = []
            if part >= 100:
                words += [ones[part // 100], 'Hundred']
                part %= 100
            if part >= 20:
                words.append(tens[part // 10])
                part %= 10
            if part > 0:
                words.append(ones[part])
            if units[i]:
                words.append(units[i])
            res += words
        return ' '.join(res)

class Solution {
    public String numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }
        String[] ones = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        String[] tens = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        String[] units = {"", "Thousand", "Million", "Billion"};
        List<String> res = new ArrayList<>();
        long[] scale = {1L, 1000L, 1000000L, 1000000000L};
        for (int i = 3; i >= 0; i--) {
            int part = (int) (num / scale[i] % 1000);
            if (part == 0) {
                continue;
            }
            List<String> words = new ArrayList<>();
            if (part >= 100) {
                words.add(ones[part / 100]);
                words.add("Hundred");
                part %= 100;
            }
            if (part >= 20) {
                words.add(tens[part / 10]);
                part %= 10;
            }
            if (part > 0) {
                words.add(ones[part]);
            }
            if (!units[i].isEmpty()) {
                words.add(units[i]);
            }
            res.addAll(words);
        }
        return String.join(" ", res);
    }
}

class Solution {
public:
    string numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }
        vector<string> ones = {"", "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"};
        vector<string> units = {"", "Thousand", "Million", "Billion"};
        vector<string> res;
        long long scale[4] = {1, 1000, 1000000, 1000000000};
        for (int i = 3; i >= 0; i--) {
            int part = (int)(num / scale[i] % 1000);
            if (part == 0) {
                continue;
            }
            vector<string> words;
            if (part >= 100) {
                words.push_back(ones[part / 100]);
                words.push_back("Hundred");
                part %= 100;
            }
            if (part >= 20) {
                words.push_back(tens[part / 10]);
                part %= 10;
            }
            if (part > 0) {
                words.push_back(ones[part]);
            }
            if (!units[i].empty()) {
                words.push_back(units[i]);
            }
            for (auto& w : words) res.push_back(w);
        }
        string out;
        for (size_t k = 0; k < res.size(); k++) {
            if (k) out += " ";
            out += res[k];
        }
        return out;
    }
};

func numberToWords(num int) string {
	if num == 0 {
		return "Zero"
	}
	ones := []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"}
	units := []string{"", "Thousand", "Million", "Billion"}
	res := []string{}
	scale := []int{1, 1000, 1000000, 1000000000}
	for i := 3; i >= 0; i-- {
		part := num / scale[i] % 1000
		if part == 0 {
			continue
		}
		words := []string{}
		if part >= 100 {
			words = append(words, ones[part/100], "Hundred")
			part %= 100
		}
		if part >= 20 {
			words = append(words, tens[part/10])
			part %= 10
		}
		if part > 0 {
			words = append(words, ones[part])
		}
		if units[i] != "" {
			words = append(words, units[i])
		}
		res = append(res, words...)
	}
	return strings.Join(res, " ")
}

var numberToWords = function(num) {
    if (num === 0) {
        return 'Zero';
    }
    const ones = ['', '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 units = ['', 'Thousand', 'Million', 'Billion'];
    const res = [];
    for (let i = 3; i >= 0; i--) {
        let part = Math.floor(num / 1000 ** i) % 1000;
        if (part === 0) {
            continue;
        }
        const words = [];
        if (part >= 100) {
            words.push(ones[Math.floor(part / 100)], 'Hundred');
            part %= 100;
        }
        if (part >= 20) {
            words.push(tens[Math.floor(part / 10)]);
            part %= 10;
        }
        if (part > 0) {
            words.push(ones[part]);
        }
        if (units[i]) {
            words.push(units[i]);
        }
        res.push(...words);
    }
    return res.join(' ');
};

function numberToWords(num: number): string {
    if (num === 0) {
        return 'Zero';
    }
    const ones = ['', '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 units = ['', 'Thousand', 'Million', 'Billion'];
    const res: string[] = [];
    for (let i = 3; i >= 0; i--) {
        let part = Math.floor(num / 1000 ** i) % 1000;
        if (part === 0) {
            continue;
        }
        const words: string[] = [];
        if (part >= 100) {
            words.push(ones[Math.floor(part / 100)], 'Hundred');
            part %= 100;
        }
        if (part >= 20) {
            words.push(tens[Math.floor(part / 10)]);
            part %= 10;
        }
        if (part > 0) {
            words.push(ones[part]);
        }
        if (units[i]) {
            words.push(units[i]);
        }
        res.push(...words);
    }
    return res.join(' ');
}

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