题目：
Given an input string, reverse the string word by word. A word is defined as a sequence of non-space characters.
The input string does not contain leading or trailing spaces and the words are always separated by a single space.
For example, Given s = "the sky is blue", return "blue is sky the".
Could you do it in-place without allocating extra space?
思路：
分两步走：1）将字符串整个翻转一遍；2）将字符串的每个单词识别出来，并翻转一遍。
代码：
class Solution { public: void reverseWords(string &s) { int length = s.length(); if (length == 0) { return; } reverseWords(s, 0, length - 1); int start = 0, end = 0; while (start < length) { end = start; while (end + 1 < length && s[end + 1] != ' ') { ++end; } reverseWords(s, start, end); start = end + 2; } } private: void reverseWords(string &s, int start, int end) { while (start < end) { char c = s[start]; s[start] = s[end]; s[end] = c; ++start, --end; } } };