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.

这个题如果用C++的stringstream字符串流来实现的话，会非常简单

void reverseWords(string &s) {
    istringstream is(s);
    string tmp;
    is >> s;
    while(is >> tmp) s = tmp + " " + s;
    if(s[0] == ' ') s = "";
}

但是，题目中对于C语言实现有更严格的要求，需要就地操作

联想到之前碰到的题目，数组旋转问题，我们是否可以用类似的思路来解决呢

对于每一个单词，都将其字母前后逆置，然后再将整个字符串逆置，这样就可以得到正确反转结果了。

但是如何处理多余的空格呢？我们可以在遍历的同时，利用快慢双指针来将空格省略掉。然后再执行逆置操作。

代码如下：

void reverseWords(string &s) {

    int i = 0, j = 0;
    int l = 0;
    int len = s.length();
    int wordcount = 0;

    while (true) {
        while (i<len && s[i] == ' ') i++;
        if (i == len) break;
        if (wordcount) s[j++] = ' ';
        l = j;
        while (i<len && s[i] != ' ') { s[j] = s[i]; j++; i++; }
        reverseword(s, l, j - 1);
        wordcount++;
    }

    s.resize(j);
    reverseword(s, 0, j - 1);
}