write a go solution for Description:
You are given a positive integer x.

You can apply the following operation to the number: remove one occurrence of any digit in such a way that the resulting number does not contain any leading zeroes and is still a positive integer. For example, 10142 can be converted to 1142, 1042, 1012 or 1014 (note that 0142 is not a valid outcome); 10 can be converted to 1 (but not to 0 since it is not positive).

Your task is to find the minimum positive integer that you can obtain from x if you can apply the aforementioned operation exactly k times.

Input Format:
The first line contains a single integer t (1<=t<=10^5) — the number of test cases.

The first line of each test case contains a single integer x (1<=x<10^500000).

The second line contains a single integer k (0<=k<|x|), where |x| is the length of the number x.

The sum of |x| over all test cases does not exceed 5*10^5.

Output Format:
For each test case, print one integer — the minimum positive number that you can obtain from x if you can apply the operation exactly k times.

Note:
None. Output only the code with no comments, explanation, or additional text.