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".

click to show clarification.

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.

分析：细节实现题，基本思路为：将各单词从s中提取出来，存放在一个vector<string>中，然后清空s并逆序遍历vector<string>将单词和空格append到s中。代码如下：

class Solution {
public:
    void reverseWords(string &s) {
        int n = s.length();
        if(n == 0) return;
        vector<string> res;
        for(int i = 0; i < n;){
            while(i < n && s[i] == ' ')i++;//skip spaces
            int j = i;
            while(j < n && s[j] != ' ')j++;
            if(i < n) res.push_back(s.substr(i,j-i));
            i = j;
        }
        s.clear();
        for(auto i = res.rbegin(); i != res.rend(); i++){
            s += *i + " ";
        }
        if(s.length() != 0) s.pop_back();
    }
};