Java入门：一些初学者需要掌握的基础算法程序——逆序输出

【问题】 将一个整数逆序输出。

【思路】 有三种方法可以将一个数逆序输出：

　　　　1）使用while循环

　　　　2）使用for循环

　　　　3）使用递归

【代码】

程序1：使用while循环

import java.util.Scanner;
class ReverseNumberWhile
{
   public static void main(String args[])
   {
      int num=0;
      int reversenum =0;
      System.out.println("Input your number and press enter: ");
      //This statement will capture the user input
      Scanner in = new Scanner(System.in);
      //Captured input would be stored in number num
      num = in.nextInt();
      //While Loop: Logic to find out the reverse number
      while( num != 0 )
      {
          reversenum = reversenum * 10;
          reversenum = reversenum + num%10;
          num = num/10;
      }

      System.out.println("Reverse of input number is: "+reversenum);
   }
}

输出结果：

Input your number and press enter:
145689
Reverse of input number is: 986541

程序2：使用for循环

import java.util.Scanner;
class ForLoopReverseDemo
{
   public static void main(String args[])
   {
      int num=0;
      int reversenum =0;
      System.out.println("Input your number and press enter: ");
      //This statement will capture the user input
      Scanner in = new Scanner(System.in);
      //Captured input would be stored in number num
      num = in.nextInt();
      /* for loop: No initialization part as num is already
       * initialized and no increment/decrement part as logic
       * num = num/10 already decrements the value of num
       */
      for( ;num != 0; )
      {
          reversenum = reversenum * 10;
          reversenum = reversenum + num%10;
          num = num/10;
      }

      System.out.println("Reverse of specified number is: "+reversenum);
   }
}

输出结果：

Input your number and press enter:
56789111
Reverse of specified number is: 11198765

程序3：使用递归

import java.util.Scanner;
class RecursionReverseDemo
{
   //A method for reverse
   public static void reverseMethod(int number) {
       if (number < 10) {
       System.out.println(number);
       return;
       }
       else {
           System.out.print(number % 10);
           //Method is calling itself: recursion
           reverseMethod(number/10);
       }
   }
   public static void main(String args[])
   {
    int num=0;
    System.out.println("Input your number and press enter: ");
    Scanner in = new Scanner(System.in);
    num = in.nextInt();
    System.out.print("Reverse of the input number is:");
    reverseMethod(num);
    System.out.println();
   }
}

输出结果：

Input your number and press enter:
5678901
Reverse of the input number is:1098765

当然，如果待逆序的这个数事先已经初始化了，就不用从键盘输入了，如：

class ReverseNumberDemo
{
   public static void main(String args[])
   {
      int num=123456789;//不从键盘输入，直接初始化为一个整数值。
      int reversenum =0;
      while( num != 0 )
      {
          reversenum = reversenum * 10;
          reversenum = reversenum + num%10;
          num = num/10;
      }

      System.out.println("Reverse of specified number is: "+reversenum);
   }
}