2.
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。
#!/usr/bin/python 2.7
# @author liuzhiqiang
# @date 2018-08
# @email lucien@pku.edu.cn
# -*- coding:utf-8 -*-
class Solution:
# s 源字符串
def replaceSpace(self, s):
# write code here
length = len(s)
space_count = 0
for i in s:
if i == ' ':
space_count += 1
new_len = length + space_count * 2
res = list(s + ' ' * (space_count * 2))
p1 = length-1
p2 = new_len-1
while p1 >= 0 and p2 >= p1:
if s[p1] == ' ':
res[p2] = '0'
p2 -= 1
res[p2] = '2'
p2 -= 1
res[p2] = '%'
p2 -= 1
else:
res[p2] = s[p1]
p2 -= 1
p1 -= 1
return ''.join(res)
该类问题也就是replace的实现，只要保证从后往前修改就行
