来自控制台的 Java 递归

Question

//当我运行程序时，什么都不打印。它就像一个无限循环 // 在输入我的字符串控制台时。// 编写一个 Java 程序，它接受来自控制台的字符串输入 // 并使用递归反转它。//反转字符串后打印结果。import java.util.Scanner;public class ReverseTry {    public static void main(String[] args) {        Scanner input = new Scanner(System.in);        System.out.println("Enter a sentence and I will reverse it");        reverseLine(input);    }    public static Scanner reverseLine(Scanner input) {        if (!input.hasNextLine()) {            return input;        } else {            //String word = input.nextLine();            //return reverseLine(input) + " " + word;            String line = input.nextLine();            reverseLine(input);            System.out.println(line);        }        return input;    }}

Answer 1

您不需要在功能中传递扫描仪。从用户那里获取输入并将其传递给递归函数。首先传递字符串，起始索引和最后一个索引。这是代码：

 public static void main(String[] args) {

    Scanner input = new Scanner(System.in);

    System.out.print("Enter a sentence and I will reverse it : ");

    String line = input.nextLine();

    String reverse = reverseLine(line, 0, line.length() - 1);

    System.out.println(reverse);

  }

Reverseline 函数接受一个输入并在指定位置交换输入字符并递归调用。

  public static String reverseLine(String input, int startIndex, int endIndex) {

    if (startIndex >= endIndex) {

      return input;

    } else {

      input = swap(input, startIndex, endIndex);

      return reverseLine(input, startIndex + 1, endIndex - 1);

    }

  }

这个函数只交换字符串的字符。

  public static String swap(String input, int start, int end) {

    char[] arr = input.toCharArray();

    arr[end] = input.charAt(start);

    arr[start] = input.charAt(end);

    return String.valueOf(arr);

  }

Answer 2

import java.util.Scanner;

public class ReverseTry {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);

        System.out.println("Enter a sentence and I will reverse it");

        reverseLine(input); //no need to pass Scanner object as input to method since the return value isn't used anyway

    }

    public static Scanner reverseLine(Scanner input) {

        if (!input.hasNextLine()) {

            return input;

        } else {

            //String word = input.nextLine();

            //return reverseLine(input) + " " + word;

            String line = input.nextLine(); 

            reverseLine(input); // after you get the line over here, 

                                //this function will be called again. 

                                //In the recursively called function, 

                                //it will again ask you for input and 

                                //once you give it, again call itself 

                                //and this will go on forever. Instead 

                                //of this endless recursive call, you 

                                //need to give a base condition on which 

                                //this method should return a useful value 

                                //without calling itself. For example, you 

                                //know that there is nothing to be 

                                //reversed in a single character string ...

            System.out.println(line);

        }

        return input;

    }

}

你可能想要像下面这样的东西，

import java.util.*;

public class ReverseTry {

public static void main(String[] args) {

    Scanner input = new Scanner(System.in);

    System.out.println("Enter a sentence and I will reverse it");

    String s = input.nextLine(); //no need to pass Scanner object into the reverseline method. Scanner object is used to read the console input only once

    s = reverseLine(s); // here is where the reversing logic goes

    System.out.println(s); // printing out the reversed string

}

public static String reverseLine(String s) {

    if(s.length() == 1 ) { //base case --> there is nothing to reverse when the string length is 1

        return s;

    }

    return s.substring(s.length()-1)+reverseLine(s.substring(0,s.length()-1)); //recursion logic

}

}