405. Convert a Number to Hexadecimal

• Difficulty: Easy

• Topics: Bit Manipulation

• Similar Questions:

Problem:

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:
26

Output:
"1a"

Example 2:

Input:
-1

Output: "ffffffff" </pre> </p>

Solutions:

class Solution {
public:
    string toHex(int num) {
        bool sign = (num >= 0);
        if (!sign) {
            num += INT_MAX;
            num += 1;
        }

        string ret;
        if (num == 0) { // this check is important!
            ret.push_back('0');
        }
        while (num > 0) {
            ret.push_back(decimalToHex(num % 16));
            num /= 16;
        }

        if (!sign) {
            while (ret.size() != 8) {
                ret.push_back('0');
            }
            ret.back() = decimalToHex(ret.back() - '0' + 8);
        }

        reverse(ret.begin(), ret.end());
        return ret;
    }

private:
    char decimalToHex(int val) {
        if (val < 10)   return '0' + val;
        else return 'a' + (val - 10);
    }

};