Leetcode 151 Reverse Words in a String

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

Example:  

Input: "the sky is blue",
Output: "blue is sky the".

Note:

• A word is defined as a sequence of non-space characters.
• Input string may contain leading or trailing spaces. However, your reversed string should not contain leading or trailing spaces.
• You need to reduce multiple spaces between two words to a single space in the reversed string.

Follow up: For C programmers, try to solve it in-place in O(1) space.

這個題的意思是將一串字元倒置。

（1）我的方法是直接將字串分離出來再進行重新的拼接

public class Solution {
    public String reverseWords(String s) {
        String[] ss = s.trim().split("\\s+");//trim().split("\\s+")
        String a = "";
        for(int i = ss.length - 1;i >= 1 ;--i){
            a += ss[i] + " ";
        }
        return a + ss[0];
    }
}

（2）最佳的是在O（1）的空間複雜度下完成的

public class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = new StringBuilder();
        int index = s.length() - 1;
        while (index >= 0) {
            if (s.charAt(index) == ' ') {
                index--;
                continue;
            }
            int start = s.lastIndexOf(' ', index) + 1;
            sb.append(' ');
            sb.append(s.substring(start, index + 1));//將空格兩邊的字元進行重新的拼接
            index = start - 1;
        }
        if (sb.length() > 0) sb.deleteCharAt(0);//這一行的意思是將最前面的空格去掉
        return sb.toString();//轉化為String
    }
}