405. Convert a Number to Hexadecimal 

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class Solution {
    public String toHex(int num) {
        if (num == 0) {
            return "0";
        }

        StringBuilder sb = new StringBuilder();
        while (num != 0) {
            int part = num & 0b1111; // 使用 0b1111 或者 15, 注意此处不是 16
            sb.append(toHexChar(part));
            num >>>= 4; // 注意此处是无符号右移，否则负数时 while 循环无法结束
        }
        return sb.reverse().toString();
    }

    private char toHexChar(int part) {
        if (part <= 9) {
            return (char) ('0' + part);
        } else {
            return (char) ('a' + part - 10);
        }
    }
}

References

405. Convert a Number to Hexadecimal