Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

**Note:**

- The length of
`num`

is less than`10002`

and will be`≥ k`

. - The given num does not contain any leading zero.

**Example 1:**

```
Input: num = "1432219", k = 3
Output: "1219"
Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.
```

**Example 2:**

```
Input: num = "10200", k = 1
Output: "200"
Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.
```

Example 3:

```
Input: num = "10", k = 2
Output: "0"
Explanation: Remove all the digits from the number and it is left with nothing which is 0.
```

## Solution

This problem is hard. Solution for each k character, remove the largest one. Reduce K for next step

Don't forget to remove leading zeroes

```
class Solution {
func removeKdigits(_ num: String, _ k: Int) -> String {
var k = k
var stack = [Character]()
for c in num {
while k > 0, let top = stack.last, top > char {
stack.removeLast()
k -= 1
}
stack.append(char)
}
stack.removeLast(k)
let result = stack.drop(while: { $0 == "0" })
return result.isEmpty ? "0" : String(result)
}
}
```