剑指Offer 05 替换空格

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。

# -*- coding:utf-8 -*-
class Solution:
    # s 源字符串
    def replaceSpace(self, s):
        n = len(s)
        whitespace = 0
        for ss in s:
            if ss ==' ':
                whitespace += 1
        ary = [' '] * (n + whitespace * 2)
        idx = n + whitespace * 2 - 1
        for i in range(n-1,-1,-1):
            if s[i] == ' ':
                ary[idx] = '0'
                idx -= 1
                ary[idx] = '2'
                idx -= 1
                ary[idx] = '%'
                idx -= 1
            else:
                ary[idx] = s[i]
                idx -= 1
        s = ''
        s = ''.join(ary)
        return s
        # 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。