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// @Title: 整数转换英文表示 (Integer to English Words)
// @Author: 15816537946@163.com
// @Date: 2021-10-11 08:51:05
// @Runtime: 4 ms
// @Memory: 2.2 MB
var (
    singles   = []string{"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"}
    teens     = []string{"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"}
    tens      = []string{"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"}
    thousands = []string{"", "Thousand", "Million", "Billion"}
)

func numberToWords(num int) string {
    if num == 0 {
        return "Zero"
    }
    sb := &strings.Builder{}
    var recursion func(int)
    recursion = func(num int) {
        switch {
        case num == 0:
        case num < 10:
            sb.WriteString(singles[num])
            sb.WriteByte(' ')
        case num < 20:
            sb.WriteString(teens[num-10])
            sb.WriteByte(' ')
        case num < 100:
            sb.WriteString(tens[num/10])
            sb.WriteByte(' ')
            recursion(num % 10)
        default:
            sb.WriteString(singles[num/100])
            sb.WriteString(" Hundred ")
            recursion(num % 100)
        }
    }
    for i, unit := 3, int(1e9); i >= 0; i-- {
        if curNum := num / unit; curNum > 0 {
            num -= curNum * unit
            recursion(curNum)
            sb.WriteString(thousands[i])
            sb.WriteByte(' ')
        }
        unit /= 1000
    }
    return strings.TrimSpace(sb.String())
}