剑指offer，题目：请实现一个函数，将一个字符串中的空格替换成“ 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。-Sword refers to the offer, please implement a function that will be a string of Spaces replace " 20". For example, when a string as We Are Happy. After replacem