剑指Offer 05 替换空格

请实现一个函数，将一个字符串中的每个空格替换成“%20”。例如，当字符串为We Are Happy.则经过替换之后的字符串为We%20Are%20Happy。

 1 # -*- coding:utf-8 -*-
 2 class Solution:
 3     # s 源字符串
 4     def replaceSpace(self, s):
 5         n = len(s)
 6         whitespace = 0
 7         for ss in s:
 8             if ss ==' ':
 9                 whitespace += 1
10         ary = [' '] * (n + whitespace * 2)
11         idx = n + whitespace * 2 - 1
12         for i in range(n-1,-1,-1):
13             if s[i] == ' ':
14                 ary[idx] = '0'
15                 idx -= 1
16                 ary[idx] = '2'
17                 idx -= 1
18                 ary[idx] = '%'
19                 idx -= 1
20             else:
21                 ary[idx] = s[i]
22                 idx -= 1
23         s = ''
24         s = ''.join(ary)
25         return s
26         # write code here

先计算出扩展后的字符串的长度然后，从后向前遍历，遇到空格，逆序依次插入对应的三个字符。