Remove K Digits - Algorithm

Use a stack. For each digit:

$1.$ While stack top > current digit and k > $0$, pop the stack (remove that digit) and decrement k.

$2.$ Push current digit. After processing, if k > $0$, remove from the end. Finally, strip leading zeros. Time: $O(n)$. Space: $O(n)$. The stack maintains an increasing sequence of digits from left to right. Each pop removes a 'peak' digit where a smaller digit follows. This ensures leftmost positions contain the smallest possible values. Common bug: forgetting to handle cases where $k$ removals finish before scanning all digits.