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=[]
        if len(num)==1 and k==1:return '0'
        n=len(num)

        for i in num:
            while st and st[-1]>i and k>0:
                k-=1
                st.pop()
            if st or i!='0':
                st.append(i)

        t=''.join(st)
        if k:t=t[:-k]
        return '0' if not t else t