402. Remove K Digits

Problem Link

Description

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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.

 

Constraints:

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

Solution

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Python3

class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        N = len(num)
        stack = []
 
        for x in num:
            while k > 0 and stack and x < stack[-1]:
                stack.pop()
                k -= 1
            
            stack.append(x)
        
        while k > 0 and stack:
            stack.pop()
            k -= 1
        
        for i in range(len(stack)):
            if stack[i] == '0': continue
 
            return "".join(stack[i:])
        
        return "0"