Given a string, num, that represents a non-negative integer and an integer k, remove exactly k digits from num so that the remaining digits form the smallest possible number.

Constraints:

• $1 \leq$ k $\leq$ num.length $\leq 10^5$

• num consists of only digits.

• num does not have any leading zeros except for the zero itself.

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We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Implement your solution in the following coding playground.