边工作边刷题：70天一遍leetcode: day 58

Integer to English Words

要点：类似roman to integer，三位一组计算，要点:

• 要从低位开始，因为不知道最高位实际是哪个位。类似的最高三位可能有短缺位，所以先reverse整个string以便3位一组loop的时候不会漏掉最高位（这个也是错误点）。

错误点：

• Ten也会表示出来，所以这位不是空
• 计算3位的逻辑不是全部if .. else，而是后两位if .. else .. 最高位是if
• 不要位数组中的字符留空格，而是利用list join时添加空格
• TypeError: can only join an iterable: " ".join(res.reverse()): 错误原因是res.reverse()是in-place function，返回的是None

test cases: 1234567, 20, 0, 1000, 100000

class Solution(object):
    def numberToWords(self, num):
        """
        :type num: int
        :rtype: str
        """
        def computeThree(num):
            one = ["", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"]
            # error 1: "Ten" instead of ""
            ten = ["Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"]
            ten_2 = ["", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"]

            res = []
            if num/100!=0:
                res.append(one[num/100]+ " Hundred")
            
            # error 3: not elif
            if (num%100)/10==1:
                res.append(ten[num%10])
            else:
                # error 2: 
                # error 5: 20 => "Twenty " vs "Twenty"
                if num%100/10 >= 2:
                    res.append(ten_2[num%100/10])
                if num%10>0:
                    res.append(one[num%10])
                
            return " ".join(res)
        
        if num==0: return "Zero"
        base = ["", " Thousand", " Million", " Billion"]
        str_num = str(num)[::-1]
        # error 4: why reverse 
        divide_list = [str_num[i:i+3][::-1] for i in xrange(0, len(str_num), 3)]

        # print divide_list
        res = []
        for i in xrange(len(divide_list)):
            three = computeThree(int(divide_list[i]))
            if three:
                res.append(three + base[i])
        res.reverse() 
        return " ".join(res)