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"

正数直接除16取余法

负数不需要额外转换，直接位操作每四位取出来转化成十六进制对应的符号

public class Solution {    public static String toHex(int num){if(num==0)return "0";Stack<Character> stack=new Stack<>();char[] carr={'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f'};StringBuilder sb=new StringBuilder();if(num>=0){while(num>0){stack.push(carr[num%16]);num/=16;}while(!stack.isEmpty())sb.append(stack.pop());}else {int mask=0xf0000000;for(int i=0;i<8;i++){int n=(num<<(i*4))&mask;int sum=0;int base=8;for(int j=0;j<4;j++){if((n&0x80000000)!=0)sum+=base;base>>=1;n<<=1;}sb.append(carr[sum]);}}int start=0;while(sb.charAt(start)=='0')start++;return sb.substring(start);}}