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:])
}