1. 题目

2. 思路

(1) 分组计算

• 每4位二进制位组成1位十六进制位，因此将整数转换成二进制位后进行分组，计算每组对应的十六进制位的值，最后拼接字符串即可。

(2) 位运算

• 与(1)的思想基本相同，计算每组对应的十六进制位的值可以使用位运算代替，将每组二进制位按照其位置右移到最低的四位，然后与0xf作与运算就可以直接得到该组的值，最后将其转换成十六进制即可。

3. 代码

import java.util.HashMap;
import java.util.Map;

public class Test {
    
    
    public static void main(String[] args) {
    
    
    }
}

class Solution {
    
    
    public String toHex(int num) {
    
    
        Map<Integer, String> map = new HashMap<Integer, String>() {
    
    {
    
    
            put(0, "0");
            put(1, "1");
            put(2, "2");
            put(3, "3");
            put(4, "4");
            put(5, "5");
            put(6, "6");
            put(7, "7");
            put(8, "8");
            put(9, "9");
            put(10, "a");
            put(11, "b");
            put(12, "c");
            put(13, "d");
            put(14, "e");
            put(15, "f");
        }};
        String s = Integer.toBinaryString(num);
        StringBuilder binary = new StringBuilder();
        int count = 4 - s.length() % 4;
        if (count < 4) {
    
    
            while (count > 0) {
    
    
                binary.append('0');
                count--;
            }
        }
        binary.append(s);
        StringBuilder res = new StringBuilder();
        count = 0;
        int index;
        int sum;
        while (count < binary.length() / 4) {
    
    
            index = 0;
            sum = 0;
            while (index < 4) {
    
    
                if (binary.charAt(count * 4 + index) == '1') {
    
    
                    sum += Math.pow(2, 3 - index);
                }
                index++;
            }
            res.append(map.get(sum));
            count++;
        }
        return res.toString();
    }
}

class Solution1 {
    
    
    public String toHex(int num) {
    
    
        if (num == 0) {
    
    
            return "0";
        }
        Map<Integer, String> map = new HashMap<Integer, String>() {
    
    {
    
    
            put(0, "0");
            put(1, "1");
            put(2, "2");
            put(3, "3");
            put(4, "4");
            put(5, "5");
            put(6, "6");
            put(7, "7");
            put(8, "8");
            put(9, "9");
            put(10, "a");
            put(11, "b");
            put(12, "c");
            put(13, "d");
            put(14, "e");
            put(15, "f");
        }};
        StringBuilder res = new StringBuilder();
        int value;
        for (int i = 7; i >= 0; i--) {
    
    
            value = (num >>> (4 * i)) & 0xf;
            if (value != 0 || res.length() != 0) {
    
    
                res.append(map.get(value));
            }
        }
        return res.toString();
    }
}