Convert a Number to Hexadecimal

class Solution {
    public String toHex(int num) {
        if (num ==0) return "0";
        StringBuilder res = new StringBuilder();
        if (num <0) {
            char[] bit = Integer.toBinaryString(num).toCharArray();
            for (int i=0; i < bit.length;i+=4) {
                int carry =(bit[i]-'0')*8 + (bit[i+1]-'0')*4 + (bit[i+2]-'0')*2 + (bit[i+3]-'0');
                res.append(carry >9? (char)('a' + (carry-10)): (char)(carry+'0'));
            }
        }
        else {
            while (num >0) {
                int carry = num % 16;
                res.append(carry >9? (char)('a' + (carry-10)): (char)(carry+'0'));
                num = num/16;
            }
        }
        return num>=0? res.reverse().toString(): res.toString();
    }
}