Reverse Words in a String II

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?

Related problem: Rotate Array

 

Runtime: 6ms

class Solution {
public:
    void reverseWords(string &s) {
        // reverse the entire string
        reverseString(s, 0, s.size() - 1);
        
        // reverse each single word
        for (int i = 0; i < s.size(); ) {
            int wordEndNext = s.find(" ", i);
            // reach to end
            if (wordEndNext == s.npos) { 
                reverseString(s, i, s.size() - 1);
                return;
            } else {
                reverseString(s, i, wordEndNext - 1);
                i = wordEndNext + 1;
            }
        }
    }
    
    void reverseString(string &s, int begin, int end) {
        for (int i = begin, j = end; i < j; i++, j--)
            swap(s[i], s[j]);
    }
};