273. Integer to English Words

recursive

unordered_map<int, string> m = {
    {1e9, "Billion"}, {1e6, "Million"}, {1e3, "Thousand"}, {100, "Hundred"},
    {90, "Ninety"}, {80, "Eighty"}, {70, "Seventy"}, {60, "Sixty"}, {50, "Fifty"}, {40, "Forty"}, {30, "Thirty"}, {20, "Twenty"},
    {19, "Nineteen"}, {18, "Eighteen"}, {17, "Seventeen"}, {16, "Sixteen"}, {15, "Fifteen"}, {14, "Fourteen"}, {13, "Thirteen"}, {12, "Twelve"}, {11, "Eleven"},
    {10, "Ten"}, {9, "Nine"}, {8, "Eight"}, {7, "Seven"}, {6, "Six"}, {5, "Five"}, {4, "Four"}, {3, "Three"}, {2, "Two"}, {1, "One"},
    {0, "Zero"}
};
class Solution {
public:
    string numberToWords(int num) {
        string res;
        vector<int> v = {1e9, 1e6, 1e3, 100}; // 100以上的数统一处理
        for (int x : v) {
            if (num >= x) {
                res.append(numberToWords(num / x)).append(" ").append(m[x]);
                num %= x;
                return num > 0 ? res.append(" ").append(numberToWords(num)) : res;
            }
        }
        if (num >= 20) { // 100以内20以上的数统一处理
            res.append(m[num - num % 10]);
            num %= 10;
            return num > 0 ? res.append(" ").append(numberToWords(num)) : res;
        }
        return m[num]; // 20以内的数直接查表
    }
};

unordered_map<int, string> m = {
    {1e9, "Billion"}, {1e6, "Million"}, {1e3, "Thousand"}, {100, "Hundred"},
    {90, "Ninety"}, {80, "Eighty"}, {70, "Seventy"}, {60, "Sixty"}, {50, "Fifty"}, {40, "Forty"}, {30, "Thirty"}, {20, "Twenty"},
    {19, "Nineteen"}, {18, "Eighteen"}, {17, "Seventeen"}, {16, "Sixteen"}, {15, "Fifteen"}, {14, "Fourteen"}, {13, "Thirteen"}, {12, "Twelve"}, {11, "Eleven"},
    {10, "Ten"}, {9, "Nine"}, {8, "Eight"}, {7, "Seven"}, {6, "Six"}, {5, "Five"}, {4, "Four"}, {3, "Three"}, {2, "Two"}, {1, "One"},
    {0, "Zero"}
};
vector<int> v = {1000000000, 1000000, 1000, 100};
class Solution {
public:
    string numberToWords(int num) {
        string res;
        if (auto it = lower_bound(begin(v), end(v), num, greater<>()); it != end(v)) {
            int x = *it;
            res.append(numberToWords(num / x).append(" ").append(m[x]));
            num %= x;
            return num > 0 ? res.append(" ").append(numberToWords(num)) : res;
        }
        if (num >= 20) {
            res.append(m[num - num % 10]);
            num %= 10;
            return num > 0 ? res.append(" ").append(numberToWords(num)) : res;
        }
        return m[num];
    }
};