# -*- coding:utf-8 -*-

class Solution:

    def ReverseSentence(self, s):

        # write code here

        if not s:

            return s

        # 思路1

        # s = s.split(' ')

        # s.reverse()

        # return " ".join(s)

        # 思路2:

        s = list(s)

        self.reverse(s, 0, len(s)-1)

        start,end = 0,0

        while start < len(s):

            if s[start] == ' ':

                start += 1

                end += 1

            elif end == len(s) or s[end] == ' ':

                self.reverse(s,start ,end -1)

                end += 1

                start = end

            else:

                end += 1

        return ''.join(s)

    def reverse(self,s,start,end):

        while start < end:

            s[start],s[end] = s[end],s[start]

            start += 1

            end -= 1

if __name__ == '__main__':

    result = Solution().ReverseSentence('student. a am I')

    print(result)