蓝桥杯基础-016-Hexadecimal_To_Octal

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本文介绍了一种从16进制字符串转换为8进制字符串的算法实现，通过查找表进行二进制辅助转换，并去除前导零。该方法适用于大范围的数值转换，避免了使用受限的整数类型。

题意

输入 n (行)

输入 n 个 16进制字符串 && 1 <= hex_chs.lehgth <= 10000 -> 16^(10000 - 1) -近似> 16^10000 -> 2^40000 >> 2^31(int) or 2^63(long) -> 中间数不能用整型->char[ ]

分别每行输出 1 个 8进制字符串

(结合16进制 = 4 bits ， 8进制 = 3 bits -> 中间数取binary_chs)

代码

import java.awt.AlphaComposite;
import java.util.Scanner;

public class Main {

	static char[] HexAlphabet = {'0','1','2','3','4',
				     '5','6','7','8','9',
				     'A','B','C','D','E','F'
								 };
	static String[] HexBin = {  "0000","0001","0010","0011",
				    "0100","0101","0110","0111",
				    "1000","1001","1010","1011",
				    "1100","1101","1110","1111",
							};
	static char[][] OctBin = {  {'0','0','0'},{'0','0','1'},{'0','1','0'},{'0','1','1'},
				    {'1','0','0'},{'1','0','1'},{'1','1','0'},{'1','1','1'},
							};
	public static void main(String[] args) {

		
		Scanner keyin = new Scanner(System.in);
		int n = keyin.nextInt();
		while (n-- > 0) {
			String hex_str = keyin.next();
			char[] hex_chs = hex_str.toCharArray();
			char[] bin_chs = hexChsToBinaryChs(hex_chs);
			char[] oct_chs = binChsToOctalChs(bin_chs);
			System.out.println(String.valueOf(oct_chs));
		}
	}
	public static int chsSearch(char[] target){
		for (int i = 0; i < OctBin.length; i++) {
			int j;
			for (j = 0; j < target.length; j++) {
				if( OctBin[i][j] != target[j] )
					break;
			}
			if(j == target.length)
				return i;
		}
		return -1;
	}
	public static char[] binChsToOctalChs(char[] bin_chs){
		if( bin_chs.length % 3 >= 1){
			bin_chs = ("0"+String.valueOf(bin_chs)).toCharArray();
			if( bin_chs.length % 3 == 2){
				bin_chs = ("0"+String.valueOf(bin_chs)).toCharArray();
			}
		}
		char[] oct_chs = new char[bin_chs.length/3];
		for (int i = bin_chs.length - 3; i >= 0 ; i -= 3) {
			char[] oct_section = { bin_chs[i],bin_chs[i+1],bin_chs[i+2] };
			int sub_oct_digit = chsSearch(oct_section);
			oct_chs[i/3] = HexAlphabet[sub_oct_digit];
		}
		int cnt_head_zero = 0;
		while(oct_chs[cnt_head_zero] == '0'){
			cnt_head_zero++;
		} 
		char[] erase_head_zero_oct_chs = new char[ oct_chs.length - cnt_head_zero];
		for (int i = 0; i < erase_head_zero_oct_chs.length; i++) {
			erase_head_zero_oct_chs[i] = oct_chs[i + cnt_head_zero];
		}
		return erase_head_zero_oct_chs;
	}
	public static char[] hexChsToBinaryChs(char[] hex_chs){
		char[] bin_chs = new char[hex_chs.length*4];
		for (int i = bin_chs.length - 4; i >= 0; i -= 4) {
			int sub_hex = binSearch(HexAlphabet, hex_chs[i/4]);
			System.arraycopy(HexBin[sub_hex].toCharArray(), 0,bin_chs, i , 4);
		}
		return bin_chs;
	}

	public static int binSearch(char[] src,char target){
		int low = 0,high = src.length - 1,mid = 0;
		do {
			mid = (low + high)/2;
			if(target == src[mid]) 
				return mid;
			else if(target > src[mid])
				low = mid + 1;
			else
				high = mid - 1;
		} while (low <= high);
		return -1;
	}
}

Summary

【两转两Search后去Zero】

i)两Search两转 -> binSearch

hex_chs + 构造HexAlphabet对照表( HexAlphabet和OctalAlphabet部分可以通用) +HexBin -> 用于 hexChsToBinaryChs

bin_chs + HexAlphabet + HexBin -> 用于 binChsToOctalChs

ii) 去Zero

在binChsToOctalCh中,将bin_chs -> oct_chs 后

iii)输出 ->sysout { String.valueOf(octal_chs) }