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 += 110         ary = [' '] * (n + whitespace * 2)11         idx = n + whitespace * 2 - 112         for i in range(n-1,-1,-1):13             if s[i] == ' ':14                 ary[idx] = '0'15                 idx -= 116                 ary[idx] = '2'17                 idx -= 118                 ary[idx] = '%'19                 idx -= 120             else:21                 ary[idx] = s[i]22                 idx -= 123         s = ''24         s = ''.join(ary)25         return s26         # 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。