Reverse Words in a String

题目：

Given an input string s, reverse the string word by word.
For example, given s = "the sky is blue", return "blue is sky the"

Example Questions Candidate Might Ask:
Q: What constitutes a word?
A: A sequence of non-space characters constitutes a word.
Q: Does tab or newline character count as space characters?
A: Assume the input does not contain any tabs or newline characters.
Q: Could the input string contain leading or trailing spaces?
A: Yes. However, your reversed string should not contain leading or trailing spaces.
Q: How about multiple spaces between two words?
A: Reduce them to a single space in the reversed string.

解答：

public String reverseWords(String s) {

    StringBuilder reversed = new StringBuilder();
    int j = s.length();

    for(int i = s.length()-1; i >= 0; i--) {
        if(s.charAt(i) == ' ') {
            j = i;
        } else if(i == 0 || s.charAt(i-1) == ' ') {
            if(reversed.length() != 0) {
                reversed.append(' ');
            }

            reversed.append(s.substring(i, j));
        }
    }

    return reversed.toString();
}

如果如何只允许使用O（1）的额外空间呢？

public void reverseWords(char[] s) {
    reverse(s, 0, s.length);
    for(int i = 0, j = 0; j <= s.length; j++) {
        if(j == s.length() || s[j] == ' ') {
            reverse(s, i, j);
            i = j + 1;
        }
    }
}

public void reverse(char[] s, int begin, int end) {
    for(int i = 0; i < (end - begin) >> 1; i++) {
        char tmp = s[begin + i];
        s[begin + i] = s[end -1 - i];
        s[end -1 - i] = tmp;
    }
}