/**
 * @Task:Given an input string,reverse the string word by word.
 * For example,
 * Given s="my name is black",
 * return "black is name my".
 */

package black.algorithm.reverse;

import java.util.Stack;

public class Reverse {

    public static String toReverse(String in){

        //存放單詞的棧
        Stack<Character> word=new
 
 Stack<Character>();
        //存放句子的棧
        Stack<Character> sentence=new Stack<Character>();
        //存放逆序後的字串
        StringBuilder out=new StringBuilder();

        //字串索引
        int i=0;

        while(i<=in.length()){
            //遇到空格
            if(i==in.length()||in.charAt(i)==' '
 
){
                if(!word.isEmpty()){
                    //將word棧中一個單詞轉移到sentence棧中
                    while(!word.isEmpty()){
                        sentence.push(word.pop());
                    }
                    //sentence棧中新增空格，即為單詞之間新增空格
                    if(!sentence.isEmpty()){
                        sentence.push(' '
 
);
                    }
                }
            }else{
                //將給定字串中的字元一個一個放進棧中
                word.push(in.charAt(i));
            }
            //字串索引自增
            i++;
        }

        while(!sentence.isEmpty()){
            out.append(sentence.pop());
        }

        return out.toString();

    }

    public static void main(String[] args) {
        String input="my name is black";
        String output=toReverse(input);
        System.out.println(output);
    }

}

執行結果：