1. Java API

import java.util.*;
public class Solution {
    public String ReverseSentence(String str) {
        int n = str.length();
        String[] words = str.split(" ");
        StringBuilder sb = new StringBuilder();
        for(int i = words.length - 1; i >= 0; i --){
            sb.append(words[i]).append(" ");
        }
        return sb.toString().trim();
    }
}

2. 双指针算法

public class Solution {
    public String ReverseSentence(String str) {
        str = str.trim(); // 删除首尾空格
        int j = str.length() - 1, i = j;
        StringBuilder res = new StringBuilder();
        while(i >= 0) {
            while(i >= 0 && str.charAt(i) != ' ') i--; // 搜索首个空格
            res.append(str.substring(i + 1, j + 1) + " "); // 添加单词
            while(i >= 0 && str.charAt(i) == ' ') i--; // 跳过单词间空格
            j = i; // j 指向下个单词的尾字符
        }
        return res.toString().trim(); // 转化为字符串并返回
    }
}