java 20:迴文串檢查，將16進制化爲10進制

1 檢查迴文字符串

A string is a palindrome if it reads the same forward and backward. The words “mom,” “dad,”
and “noon,” for instance, are all palindromes.

import  java.util.Scanner;
public class CheckPalin
{
	public static void main(String [] args)
	{
		Scanner input=new Scanner(System.in);
		String s=input.nextLine();
		int j=s.length()-1;
		int i=0;

		if(ifPar(s)) 
			System.out.println("the string  "+s+" is a palindromes");
		else 
			System.out.println("the string  "+s+" is not a palindromes");
	}
	public static boolean ifPar(String s)
	{
		int i=0;
		int j=s.length()-1;
		while(i<j)
		{
			if(s.charAt(i)!=s.charAt(j)) return false;
			j--;
			i++;
		}
		return true;
		
	}
}

這裏注意String 中一些常見的一些方法，例如charAt(index) 返回index位置的字符，他相對是是indexof(char c)返回char c在字符串中的index

2 十六進制轉爲十進制

 一個十六進制 h_nh_n-1……h₁h_{0轉爲十進制是}

H_n*16ⁿ+h_n-1*16^n-1+…….h₁*16+h₀

_{可以化簡爲} 

(…((h_n*16+h_n-1)*16+h_n-2)*16+…..+h₁)*16+h_{0 這樣可以用一個變量來存放結果，從index=0開始遍歷，將讀到的字符化爲int(用n存放)， 然後result=result*16+n}

<span style="font-family: Arial, Helvetica, sans-serif;">import java.util.Scanner;</span>
public class HexToDec
{
	public static void main(String [] args)
	{
		Scanner input = new Scanner(System.in);
		String hexS=input.nextLine();
		System.out.println("Hex:"+hexS+" to Dec: "+hexToDec(hexS));
		
	}
	public static int hexToDec(String s)
	{
		int result=0;
		for(int i=0;i<s.length();i++)
		{
			int n=hexCharToDec(s.charAt(i));
			result=result*16+n;
		}
		return result;
	}
	public static int hexCharToDec(char a)
	{
		if(a>='A' && a<='F') return a-'A'+10;
		else return a-'0';
	}
}