1. 调用函数
    class Solution:
 # s 源字符串
 def replaceSpace(self, s):
     # write code here
     if not s:
         return ''
     return  '%20'.join(s.split(' '))
    # return  s.replace(' ', '%20')

2.书上的做法

# -*- coding:utf-8 -*-
class Solution:
    # s 源字符串
    def replaceSpace(self, s):
        # write code here
        k = len(s)
        n = 0
        for i in s:
            if i == ' ':
                n += 1
        kk = k + 2*n
        ss = [0]*kk
        i = k - 1
        while i >= 0:
            if s[i] == ' ':
                ss[i+2*n] = '0'
                ss[i-1+2*n] = '2'
                ss[i-2+2*n] = '%'
                i -= 1
                n -= 1
            else:
                ss[i+2*n] = s[i]
                i -= 1 
        return ''.join(ss)

一道相似题https://leetcode-cn.com/problems/merge-sorted-array/