Learning On the Way

402. Remove K Digits

Given string num representing a non-negative integer num, and an integer k, return the smallest possible integer after removing k digits from num.

class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        st = []
        for n in num:
            while st and k and st[-1] > n:
                st.pop()
                k -= 1
            
            if st or n is not '0': # prevent leading zeros
                st.append(n)
                
        if k: # not fully spent
            st = st[0:-k]
            
        return ''.join(st) or '0'