题目：

题解：

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