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?

思路：
要求是否可以不需要额外空间，即只在原来的char数组上操作。
首先可以把整个char数组反转，然后再对每个单词进行反转。
反转可以用一个函数reverse实现。
找每个需要反转的单词可以通过双指针结合游标的方式实现。

public void reverseWords(char[] str) {
    if (str == null || str.length == 0) {
        return;
    }

    reverse(str, 0, str.length - 1);

    //寻找单词的双指针，i是游标
    int wordStart = 0, wordEnd = 0;
    for (int i = 0; i < str.length; i++) {
        if (str[i] != ' ') {
            wordEnd = i;
        }
        if (str[wordStart] == ' ' && str[i] != ' ') {
            wordStart = i;
        }
        if (str[i] == ' ' || i == str.length - 1) {
            if (str[wordStart] != ' ' && str[wordEnd] != ' ') {
                reverse(str, wordStart, wordEnd);
                wordStart = wordEnd = i + 1;
            }
        }
    }
}

private void reverse(char[] str, int start, int end) {
    while (start < end) {
        char tmp = str[start];
        str[start++] = str[end];
        str[end--] = tmp;
    }
}