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:

• All letters in hexadecimal (a-f) must be in lowercase.
• 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.
• The given number is guaranteed to fit within the range of a 32-bit signed integer.
• You must not use any method provided by the library which converts/formats the number to hex directly.

Example 1:

Input:
26

Output:
"1a"

Example 2:

Input:
-1

Output:
"ffffffff"

1. Analyse

Convert the number to binary expression and then convert to hexadecimal.

2. AC Code

(16 Lines)

class Solution {
public:
    string toHex(int num) {
        map<int,string> mp; 
        mp[0]="0",  mp[1]="1",  mp[2]="2",  mp[3]="3",  mp[4]="4",  mp[5]="5",  mp[6]="6",  mp[7]="7",  
        mp[8]="8",  mp[9]="9",  mp[10]="a", mp[11]="b", mp[12]="c", mp[13]="d", mp[14]="e", mp[15]="f";

        string ret;
        for( int i=0; i<8 && num; i++ ){
            ret = mp[(unsigned int)num%16] + ret;     
            num >>= 4;
        }
            
        return ret.empty() ? "0" : ret;
    }   
};

There is a trick here.

//In C++
-1 % 16 = -1
-1 / 16 = 0

//In Python
-1 % 16 = 15
-1 / 16 = -1

So I can just use bit operation >> rather than /.