Minimize a Number After Removing Digits

# Minimize a Number After Removing Digits Given a decimal string `num` representing a nonnegative integer and an integer `k`, delete exactly `k` digits. Keep the relative order of the digits that remain, and return the smallest possible resulting number as a decimal string. The returned string must have no leading zeros. Return `"0"` when the remaining digits are all zeros or when all digits were deleted. ### Function `string removeKdigits(string num, int k)` ### Example 1 ```text Input: num = "540321", k = 2 Output: "321" ``` Deleting `5` and `4` leaves `"0321"`, which is written as `"321"`. ### Example 2 ```text Input: num = "1111", k = 3 Output: "1" ``` ### Constraints - `1 <= k <= num.length <= 100000`. - `num` contains only decimal digits. - `num` has no leading zeros unless it is exactly `"0"`. ### Clarifications Delete occurrences, not all copies of a chosen digit. Formatting away leading zeros does not change the requirement to delete exactly `k` original digits. Do not rearrange the retained digits. The number can be much larger than a machine integer. ### What a Strong Answer Covers - Exactly the required number of deletions, including repeated digits. - Leading zeros and deleting the entire input. - Why no other retained sequence produces a smaller value. - Time and storage bounds for a hundred-thousand-digit input.