518. Find Smallest Possible Number
Given a non-negative integer num represented as a string and an integer k, find the smallest possible number that can be obtained by removing k digits from num.
The first line contains the string num. The second line contains the integer k.
Print the smallest possible integer as a string.
1 <= k <= num.length <= 10^5 num is a string consisting only of digits num does not have any leading zeros except for the zero itself