Remove k digits to get the smallest price
Asked at DRW, Five Rings
Given a number as a digit string, delete exactly k digits so the remaining digits (in their original order) form the smallest possible number. Do not reorder digits, and strip any leading zeros from the result.
num = "1432219", k = 3 -> "1219"
num = "10200", k = 1 -> "200"
num = "10", k = 2 -> "0"
Return the smallest number achievable after removing exactly k digits.
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.