题目:

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

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

• 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.

解题思路：

       从头到尾将每个单词提取出来，反向赋值就行了。由于栈具有“先进后出”的性质，所以我用的栈实现。

class Solution {
public:
    void reverseWords(string &s) {
        stack<string> stk;        
        istringstream in(s);
        string tmp;
        while( in >> tmp)
            stk.push(tmp);
        s = "";
        while(!stk.empty())
        {
            s +=stk.top();
            if(stk.size()!=1)
                s +=" ";
            stk.pop();
        }
    }
};