Intuition

Just use the transformation algorithm from decimal to hexadecimal

Approach

Simulation by doing the following:

compute remain = num % 16, add remain to the result (push_back)
update num = (num - remainder) / 16
repeat step 1 and step 2 until num is 0

Notice that when num < 0, we need use its complement.

Complexity

Time complexity: the loop depends on the number of digits of num.
$$O(\log n)$$
Space complexity:
$$ O(1) $$

Code

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class Solution {
public:
    string toHex(long num) {
        if (num == 0)   return "0";
        if (num < 0)    num = INT_MAX + num + 2 + INT_MAX;
        string result;
        while (num) {
            int divide = num % 16;
            num = (num - divide) / 16;
            if (divide < 10)    result += ('0' + divide);
            else result += ('a' + (divide - 10));
        }
        reverse(result.begin(), result.end());
        return result;
    }
};

405. Convert a Number to Hexadecimal

Convert a decimal number to hexadecimal.

Intuition

Approach

Complexity

Code