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Remove K Digits

Problem Statement

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

Leetcode link

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.

Constraints:

  • 1 <= k <= num.length <= 105
  • num consists of only digits.
  • num does not have any leading zeros except for the zero itself.

Code

Python Code
class Solution:
def removeKdigits(self, num: str, k: int) -> str:
st = list()
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'