practice_402.py

"""
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.

Example 1:
    Input: num = "1432219", k = 3
    Output: "1219"
    Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.
Example 2:
    Input: num = "10200", k = 1
    Output: "200"
    Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.
Example 3:
    Input: num = "10", k = 2
    Output: "0"
    Explanation: Remove all the digits from the number and it is left with nothing which is 0.
"""
class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        stack = []
        for c in num:
            while k>0 and stack and c<stack[-1]:
                stack.pop()
                k -= 1
            stack.append(c)
        stack = stack[:len(stack)-k]
        return str(int("".join(stack))) if len(stack)>0 else "0"