Remove K Digits

Given string num representing a non-negative integer and an integer k, return the smallest possible integer after removing k digits from num.

Key Insight: To minimize the number, we want smaller digits at the front. Use a monotonic increasing stack: whenever we see a digit smaller than the stack top, pop larger digits (if we still have removals left).

Greedy Strategy:

1. Iterate through digits left to right
2. While stack is non-empty, stack top > current digit, and k > 0: pop the stack (remove that digit)
3. Push the current digit
4. If k > 0 after processing all digits, remove from the end
5. Strip leading zeros and handle edge case of empty result

Why this works: A larger digit followed by a smaller digit creates an opportunity - removing the larger digit shifts smaller digits left, reducing the overall number.