面试题05. 替换空格
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https://leetcode-cn.com/problems/ti-huan-kong-ge-lcof/

请实现一个函数，把字符串 s 中的每个空格替换成"%20"。


示例 1：
::

    输入：s = "We are happy."

    输出："We%20are%20happy."


限制：

    0 <= s 的长度 <= 10000

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.. note::

    - 时间复杂度：O(n)
    - 空间复杂度：O(n)

.. code:: python

    class Solution(object):
        def replaceSpace(self, s):
            """
            :type s: str
            :rtype: str
            """
            rs  = []
            for c in s:
                if c == ' ':
                    rs.append('%20')
                else:
                    rs.append(c)
            return ''.join(rs)