StringBuilder + Stack

import java.util.*;
public class Solution {
    public String ReverseSentence(String str) {
        Stack<String> s = new Stack<>();
        String[] tmp = str.split(" ");
        for (int i = 0; i < tmp.length; i++) {
            s.push(tmp[i]);
        }
        StringBuilder result = new StringBuilder();
        while (!s.isEmpty())
            result.append(s.pop() + " ");
		// 删除最后一个空格
        result.deleteCharAt(result.length() - 1);
        return result.toString();
    }
}