題目：

請實現一個函式，將一個字串中的空格替換成“%20”。例如，當字串為We Are Happy.則經過替換之後的字串為We%20Are%20Happy。

1.使用python自帶的replace方法：

# -*- coding:utf-8 -*-
# -*- python3.6.6 -*-
# -*- JluTiger  -*-
# -*- 替換空格 -*-
class Solution:
    # s 源字串
    def replaceSpace(self, s):
        # write code here
        return s.replace("
 
 ","%20")

if __name__=="__main__":
    s="we are famliy"
    answer = Solution().replaceSpace(s)
    print(answer)

2.對空格使用split，再使用“%20”連線：

class Solution:
    # s 源字串
    def replaceSpace(self, s):
        # write code here
        return '%20'.join(s.split(' '))
if __name__=="__main__":
    s
 
="we are famliy"
    answer = Solution().replaceSpace(s)
    print(answer)

3.由於替換空格後，字串長度需要增大。先掃描空格個數，計算字串應有的長度，從後向前一個個字元複製（需要兩個指標）。這樣避免了替換空格後，需要移動的操作：

class Solution:
    # s 源字串
    def replaceSpace(self, s):
        # write code here
        #計算有多少個空格
        num_space = 0
        for i in s:
            
 
if i == ' ':
                num_space += 1
        #新的字串的長度
        new_length = len(s) + 2 * num_space
        index_origin = len(s) - 1
        index_new = new_length - 1
        #新的字串
        new_string = [None for i in range(new_length)]
        #按照從後往前的順序將%20填入
        while index_origin >= 0 & (index_new > index_origin):
            if s[index_origin] == ' ':
                new_string[index_new] = '0'
                index_new -= 1
                new_string[index_new] = '2'
                index_new -= 1
                new_string[index_new] = '%'
                index_new -= 1
            else:
                new_string[index_new] = s[index_origin]
                index_new -= 1
            index_origin -= 1
        return ''.join(new_string)

if __name__=="__main__":
    s="we are famliy"
    answer = Solution().replaceSpace(s)
    print(answer)