402 - Remove K Digits

2020/05/13

leetcode

Problem

Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

Note: The length of num is less than 10002 and will be ≥ k. The given num does not contain any leading zero.

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.

Solution

Stack

Corner cases:

‘1234567’ k = 3: k will not be reduced to 0. So stack[:-k or None] ‘10200’ k = 1: k can be reduced to 0. stack[:-0] will result in []. So stack[:-k or None] ‘1000000’ k = 1: ……lstrip(‘0’) will result 0. So we have to add or '0'

class Solution:
    def removeKdigits(self, num, k):
        stack = []
        for c in num:
            while k and stack and stack[-1] > c:
                stack.pop()
                k -= 1
            stack.append(c)

        return ''.join(stack[:-k or None]).lstrip('0') or '0'