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 * unittoEnglish(curNum)sb.WriteString(thousands[i])sb.WriteByte(' ')}unit /= 1000}return strings.TrimSpace(sb.String())
}