一般第一反应想到直接用python自带的replace就done了
classSolution:
    def replaceSpace(self , s ):
        # write code here
        returns.replace(" ","%20")
后面想到可能他想考的是辅助栈吧

classSolution:
    def replaceSpace(self , s ):
        # write code here
        stack = []
        forc in s:
            ifc == ' ': stack.append('%20')
            else: stack.append(c)
        return''.join(stack)