第六版Cracking the Coding Interview题目1.3，题目如下：

URLify: Write a method to replace all spaces in a string with '%20': You may assume that the string has sufficient space at the end to hold the additional characters, and that you are given the "true" length of the string. (Note: If implementing in Java, please use a character array so that you can perform this operation in place.)

EXAMPLE
Input: "Mr John Smith    "， 13
Output: "Mr%20John%20Smith"

这道题目要求将字符串中所有空格替换为'%20'，其实在Python中直接使用字符串的replace方法即可。但题目中明确表示需要使用in place的操作，不难想到需要从字符串尾端向前进行操作。由于在Python中字符串是不可变的，先把它转换为list之后模拟这个问题并实现如下：

def URLify(str_, trueLength):
    index = trueLength + str_[:trueLength].count(' ') * 2
    result = list(str_)
    rear = index - 1
    for head in xrange(trueLength - 1, 0, -1):
        if result[head] == ' ':
            result[rear - 2:rear + 1] = '%20'
            rear -= 3
        else:
            result[rear] = result[head]
            rear -= 1
    return ''.join(result[:index])