leetcode - [1]Reverse Words in a String

Question: Reverse Words in a String

Given an input string, reverse the string word by word.

For example,
Given s = "the sky is blue",
return "blue is sky the".

Clarification:

• What constitutes a word?
  A sequence of non-space characters constitutes a word.
• Could the input string contain leading or trailing spaces?
  Yes. However, your reversed string should not contain leading or trailing spaces.
• How about multiple spaces between two words?
  Reduce them to a single space in the reversed string.

思路：每个单词按顺序翻转，组合每个单词，对组合的句子再翻转。

例子： s = "ab  cd"

<1>s的单词翻转后是"ba dc",记为_s

<2>将_s再翻转后是"cd ab"