Reverse words

Given an input string, reverse the string word by word.

For example,
Given s = "the sky is blue",
return "blue is sky the".

click to show clarification.

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.

值得注意的就是stack的操作，不要用stack.size() 和stack.pop()一起，size 会变！

public String reverseWords(String s) {Stack<String> stack = new Stack<String>();for (int i = 0; i < s.length();) {while (i < s.length() && s.charAt(i) == ' ') {i++;}int j = i;for (; j < s.length() && s.charAt(j) != ' '; j++) {;}stack.add(s.substring(i, j));i = j;}        String result = "";        while (!stack.isEmpty()) { //<span style="color:#ff0000;">不要用stack.size()的for循环，因为采用了stack.pop()操作</span>

            result = result.isEmpty() ? stack.pop() : result + " " + stack.pop();        }return result;}