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:

```
func removeKdigits(num string, k int) string {
if k == 0 {
return num
}
stack := []byte{}
// lenN := lne(num)
for _ ,v := range num {
for len(stack) > 0 && k > 0 && (stack[len(stack) - 1] > byte(v)){
k--
stack = stack[:len(stack) - 1]
}
stack = append(stack, byte(v))
}
for k > 0 {
stack = stack[:len(stack) - 1]
k--
}
firstNoneZero := -1
for k,v := range stack {
if v > '0' {
firstNoneZero = k
break
}
}
if len(stack) == 0 || firstNoneZero == -1 {
return "0"
}
return string(stack[firstNoneZero:])
}
```