递归 Java语言程序设计 第十八章 （示例代码1）

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//使用递归计算阶乘
import java.util.Scanner;

public class ComputeFactorial
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter a nonnegative integeer: ");
        int n = input.nextInt();

        System.out.println("Factorial of " + n + " is " + factorial(n));
    }

    public static long factorial(int n)
    {
        if (n == 1)
        {
            return 1;
        }
        else
        {
            return n * factorial(n - 1);
        }
    }
}

//递归计算斐波那契数列，费时
import java.util.Scanner;

public class ComputeFibonacci
{
    public static void main(String[] args)
    {
        long start = System.currentTimeMillis();
        Scanner input = new Scanner(System.in);
        System.out.print("Enter an index for a number at Fibonacci number : ");
        int index = input.nextInt();

        System.out.println("The fibonacci number at index " + index + " is " + fib(index));
        long finish = System.currentTimeMillis();
        System.out.println("Use " + (finish - start) + " millis");
    }

    public static long fib(long index)
    {
        if (index == 0 )
        {
            return 0;
        }
        if (index == 1)
        {
            return 1;
        }
        else
            return fib(index - 1) + fib(index - 2);
    }
}
//Enter an index for a number at Fibonacci number : 40
//The fibonacci number at index 40 is 102334155
//Use 1540 millis

//非递归计算斐波那契数列
import java.util.Scanner;

public class  ComputeFibonacciNoRecursive
{
    public static void main(String[] args)
    {
        long start = System.currentTimeMillis();
        Scanner input = new Scanner(System.in);
        System.out.print("Enter an index for a number at Fibonacci number : ");
        int index = input.nextInt();

        System.out.println("The fibonacci number at index " + index + " is " + fib(index));
        long finish = System.currentTimeMillis();
        System.out.println("Use " + (finish - start) + " millis");
    }

    public static long fib(long index)
    {
        long first = 0, second = 1;
        for (long i = 0; i < index; i++)
        {
            long  next = first + second;
            first = second;
            second = next;
        }

        return first;
    }
}
//Enter an index for a number at Fibonacci number : 40
//The fibonacci number at index 40 is 102334155
//Use 200 millis

//使用尾递归重写斐波那契数列
import java.util.Scanner;

public class ComputeFibonacciTailRecursion
{
    public static void main(String[] args)
    {
        long start = System.currentTimeMillis();
        Scanner input = new Scanner(System.in);
        System.out.print("Eneter an index for a fibonacci number: ");
        int index = input.nextInt();

        System.out.println("The Fibonacci number at index " + index + " is " + fib(index));
        long finish = System.currentTimeMillis();
        System.out.println("Use " + (finish - start) + " millis");
    }

    public static long fib(int index)
    {
        long result = 0;
        return fib(index, 0, 1);
    }
    //每次只是关注接下来的两个连续的斐波那契数，
    //寻找次数在一次次减少，直到找到最后一个，也就是要找的那个
    public static long fib(int index, long start, long next)
    {
        if (index == 0)
        {
            return start;
        }
        if (index == 1)
        {
            return next;
        }
        else
        {
            long temp = start;
            start = next;
            next = temp + start;
            return fib(index - 1, start, next);
        }
    }
}
//Eneter an index for a fibonacci number: 40
//The Fibonacci number at index 40 is 102334155
//Use 160 millis

//回文串
import java.util.Scanner;

public class RecursivePalindromeUsingSubstring
{
    public static void main(String[] args)
    {
        long start = System.currentTimeMillis();
        System.out.println("Is moon a palindrome?" + isPalindrome("moon"));
        System.out.println("Is noon a palindrome?" + isPalindrome("noon"));
        System.out.println("Is aba a palindrmoe?" + isPalindrome("aba"));
        System.out.println("Is ab a palindrome?" + isPalindrome("ab"));
        long finish = System.currentTimeMillis();
        System.out.println("Use " + (finish - start) + " millis");
    }
    //判断回文数，一个字符串的第一个和最后一个字符不匹配那就是false
    //如此判断下去，到最中间一个还没有false那就是true
    //每次一个递归，都减少已经判断的第一个和最后一个字符
    public static boolean isPalindrome(String s)
    {
        if (s.length() <= 1)
        {
            return true;
        }
        else if (s.charAt(0) != s.charAt(s.length() - 1))
        {
            return false;
        }
        else
            return isPalindrome(s.substring(1, s.length() - 1));
    }
}

//改进回文串
import java.util.Scanner;

public class RecursivePalindrome
{
    public static void main(String[] args)
    {
        long start = System.currentTimeMillis();
        System.out.println("Is moon a palindrome?" + isPalindrome("moon"));
        System.out.println("Is noon a palindrome?" + isPalindrome("noon"));
        System.out.println("Is aba a palindrmoe?" + isPalindrome("aba"));
        System.out.println("Is ab a palindrome?" + isPalindrome("ab"));
        long finish = System.currentTimeMillis();
        System.out.println("Use " + (finish - start) + " millis");
    }
    //判断回文数，一个字符串的第一个和最后一个字符不匹配那就是false
    //如此判断下去，到最中间一个还没有false那就是true
    //每次一个递归，都减少已经判断的第一个和最后一个字符
    public static boolean isPalindrome(String s)
    {
        return isPalindrome(s, 0, s.length() - 1);
    }

    public static boolean isPalindrome(String s, int low, int high)
    {
        if (low <= high)
        {
            return true;
        }
        else if (s.charAt(low) != s.charAt(high))
        {
            return false;
        }
        else
            return isPalindrome(s, low + 1, high - 1);
    }
}