Reverse Words in a String —— Leetcode

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.

class Solution {
public:
    void reverseWords(string &s) {
        if(s.empty()) {
            s.clear();
            return ;
        }
        
        int i = 0;
        stack<string> st;
        
        while(i<s.size()) {
            //将i置于每个单词首字母位置
            while(s[i]==' ') i++;
            if(i==s.size())   break;
            //将j置于单词末尾处后一位
            int j = i;
            while(s[j]!=' ' && j<s.size())    j++;
            //将该单词添加入stack
            st.push(s.substr(i, j-i));

            i = j;
        }
        //翻转后的单词
        s.clear();
        while(!st.empty()) {
            s += st.top()+" ";
            st.pop();
        }
        s = s.substr(0, s.size()-1);
    }
};