方法一：借助栈的先入后出反转字符串（栈+StringBuilder+String[]）

public String ReverseSentence(String str) {
        //if(str==null)//这种字符串判空方式会报错
        //if(str.isEmpty())//字符串判空0
        //if(str.trim().equals(""))//字符串判空1
        if(str.length()<=0)return str;//字符串判空2
        String[] sArr = str.trim().split(" ");//分割字符串用数组接收
        Deque<String> stk = new LinkedList<>();//用Deque代替Stack
        for(int i =0;i<sArr.length;i++){
            stk.push(sArr[i]);
        }
        StringBuilder sb = new StringBuilder();
        while(!stk.isEmpty()){//栈的判空
            String s = stk.pop();
            sb.append(s+" ");
        }
        return sb.toString().trim();
    }

方法二：无需栈，字符串数组遍历时直接倒序加入StringBuilder

 public String ReverseSentence(String str) {
        //if(str==null)//这种字符串判空方式会报错
        //if(str.isEmpty())//字符串判空0
        //if(str.trim().equals(""))//字符串判空1
        if(str.length()<=0)return str;//字符串判空2
        String[] sArr = str.trim().split(" ");
        StringBuilder sb = new StringBuilder();
        for(int i =0;i<sArr.length;i++){
            sb.append(sArr[sArr.length-1-i]);//无需栈，直接倒序加入StringBuilder
            sb.append(" ");
        }
        return sb.toString().trim();
    }