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".

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

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.

思路：in-place方法。

先将字符串s反转，去除多余的空格，然后把每个单词再反转一次。 

class Solution {
public:
    void reverseWords(string &s) {
        int l = 0, r = s.size() - 1;
        while (l < r)
            swap(s[l++], s[r--]);
        for (int i = 0; i < s.size(); i++)
        {
            if (i == 0)
                while (i < s.size() && s[i] == ' ')//eliminate spaces at the beginning
                    s.erase(s.begin());
            else if (s[i - 1] == ' ')
                while (i < s.size() && s[i] == ' ')//eliminate duplicate spaces in the string
                    s.erase(s.begin() + i);
        }
        while (s.back() == ' ')//eliminate spaces at the end of the string
            s.erase(s.end() - 1);
        for (int i = 0, n = s.size(); i < n;)
        {
            for (r = i + 1; r < n && s[r] != ' '; r++);//find the last character of the current word
            l = i, r--;//reverse the current word
            i = r + 2;
            while (l < r)
                swap(s[l++], s[r--]);
        }
    }
};