Given an input string s, reverse the order of the words.

A word is defined as a sequence of non-space characters. The words in s will be separated by at least one space.

Return a string of the words in reverse order concatenated by a single space.

Note that s may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.

Example 1:

Input: s = "the sky is blue"
Output: "blue is sky the"

Example 2:

Input: s = "  hello world  "
Output: "world hello"
Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3:

Input: s = "a good   example"
Output: "example good a"
Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

<解題>

1. 先創建一個 StringBuilder 物件 sb，用於構建反轉後的字串
2. 從最後一個字符開始向前遍歷原始字串
3. 把空格先排除
4. 找到單字後從最後一個開始找回

public class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = new StringBuilder();
        int n = s.length();
        int i = n - 1;
        while(i >= 0) {
            if (s.charAt(i) == ' ') {
              i--;
              continue; 
              }
            int j = i;
            while(j >= 0 && s.charAt(j) != ' ') j--;
            sb.append(" ");
            sb.append(s.substring(j + 1, i + 1));
            i = j;
        }
        if (sb.length() > 0) sb.deleteCharAt(0);
        return sb.toString();  
    }
}