Reverse Words in a String--not finished yet

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

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

Update (2015-02-12):
For C programmers: Try to solve it in-place in O(1) space.

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.

 

Analyse: For my first thought, I will reverse the string first, then every word in the string has been reversed. Then I find each word, and reverse them. Space complexity is O(1). Time complexity is O(1). 

class Solution {
public:
    void reverse(string& s, int start, int end){
    if(start >= end) return;
    for(int i = start, j = end; i < j; i++, j--)
        swap(s[i], s[j]);
}


void reverseWords(string& s) {
    if(s.size() < 2) return;
    
    //first inverse the string
    //every word has been reversed
    reverse(s, 0, s.size() - 1);
    
    //for each word, reverse the word back
    int start = 0;
    for(int i = 0; i < s.size(); i++){
        while(s[i] != ' ') i++;
        if(i >= s.size()) i = s.size();
        reverse(s, start, i - 1);
        start = i + 1;
    }
    
    for(int i = 0; i < s.size(); i++)
        cout<<s[i];
    
}
};