Given a non-negative integer S represented as a string, remove K digits from the number so that the new number is the smallest possible.
Note : The given num does not contain any leading zero.

Example 1:

Input:
S = "149811", K = 3
Output: 111
Explanation: Remove the three digits 
4, 9, and 8 to form the new number 
111 which is the smallest.

​Example 2:

Input: 
S = "1002991", K = 3
Output: 21
Explanation: Remove the three digits
1(leading one), 9, and 9 to form the 
new number 21(Note that the output 
must not contain leading zeroes) 
which is the smallest.

Your Task:
You don't need to read input or print anything. Your task is to complete the function removeKdigits() which takes the string S and an integer K as input and returns the new number which is the smallest possible.

Expected Time Complexity: O(|S|).
Expected Auxiliary Space: O(|S|).

Constraints:
1<=|S|<=10002
1<=K<=|S|