Problem Description:

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?

Since this problem has guaranteed that the string does not contain all the leading and trailing spaces and all words are separated by a single space, it will be much easier to be solved in space. The idea is to reverse the whole string first. Then we visit the string from left to right, each time we meet a space, we reverse the immediate word before it.

The code is as follows.

1 class Solution { 2 public: 3     void reverseWords(string &s) { 4         reverse(s.begin(), s.end()); 5         s += ' '; 6         int i = 0, j = 0, n = s.length(); 7         while (i < n && j < n) { 8             while (j < n && s[j] != ' ') j++; 9             if (j < n) {10                 reverseBetween(s, i, j - 1);11                 i = j + 1;12                 j = i;13             }14         }15         s.resize(n - 1);16     }17 private:18     void reverseBetween(string &s, int i, int j) {19         while (i < j)20             swap(s[i++], s[j--]);21     }22 };

Note that we append a space to the reversed string to facilitate the detection of the last word.