LeetCode-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.

Solution:

Code:

class Solution {public:    void reverseWords(string &s) {        int length = s.size();        stack<string> stk;        int start;        bool word = false;        for (int i = 0; i < length; ++i)            if (s[i] == ' ' && word) {                stk.push(s.substr(start, i-start));                word = false;            } else if (s[i] != ' ' && !word) {                word = true;                start = i;            }        //attention        if (word) stk.push(s.substr(start, length-start));        if (stk.empty()) {            s = "";            return;        } else {            s = stk.top();            stk.pop();            while (!stk.empty()) {                s += ' ' + stk.top();                stk.pop();            }            return;        }    }};