405. Convert a Number to Hexadecimal

Given an integer, write an algorithm to convert it to hexadecimal. For negative integer, two’s complement method is used.

Note:

1. All letters in hexadecimal (a-f) must be in lowercase.
2. The hexadecimal string must not contain extra leading 0s. If the number is zero, it is represented by a single zero character '0'; otherwise, the first character in the hexadecimal string will not be the zero character.
3. The given number is guaranteed to fit within the range of a 32-bit signed integer.
4. You must not use any method provided by the library which converts/formats the number to hex directly.

Example 1:

Input:26Output:"1a"

Example 2:

Input:-1Output:"ffffffff"

把一个十进制数转换成用字符串表示的16进制字符串，不需要过多的考虑负数问题，因为负数在做位操作时已经自动转换成了补码+1的形式直接进行正常操作即可

每四位取一次，用一个数组表示对应关系

public class Solution {   public String toHex(int num) {if(num<10 && num>=0) return Integer.toString(num);char[] hex = {'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f'};StringBuilder sb = new StringBuilder();for(int i=0;i<8 && num!=0;i++) {sb.insert(0,hex[num & 15]);num = num>>4;}return sb.toString();}}