Algorithm

Use a stack to store digits of the resulting number
Iterate through each digit of the input string
While the stack is not empty, the current digit is smaller than the top of the stack, and k is greater than 0, pop from the stack and decrement k
Push the current digit onto the stack
If k is still greater than 0 after the iteration, remove digits from the end of the stack
Collect the remaining digits from the stack to form the result
Remove any leading zeroes from the result
If the result is empty after removing zeroes, return "0"