# 405. Convert a Number to Hexadecimal

Given an integer, write an algorithm to convert it to hexadecimal. For negative integer, [two’s complement](https://en.wikipedia.org/wiki/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 `0`s. 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"
```

{% hint style="info" %}
要限制長度是因為signed int如果right shift，開頭的bit不會自動變成0，如果本來是negative number，那麼開頭補進來的數就會是1。但while loop的終止條件是num變成0，所以需要特別限制res的長度。
{% endhint %}

```cpp
string toHex(int num) { // time: O(1); space: O(1)
    string hex = "0123456789abcdef";
    if (num == 0) return "0";
    string res;
    while (num && res.length() < 8) {
        res = hex[num & 0xf] + res;
        num >>= 4;
    }
    return res;
}
```

{% hint style="info" %}
如果用unsigned int就沒right shift後補進來的開頭bits的問題。
{% endhint %}

```cpp
string toHex(int num) { // time: O(1); space: O(1)
    string hex = "0123456789abcdef";
    if (num == 0) return "0";
    string res;
    unsigned int n = num;
    while (n) {
        res = hex[n & 0xf] + res;
        n >>= 4;
    }
    return res;
}
```
