LeetCode 402. Remove K Digits

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题目

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:

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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:

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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:

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Input: num = "10", k = 2
Output: "0"
Explanation: Remove all the digits from the number and it is left with nothing which is 0.

思路

维护一个保持递增顺序的栈。

代码

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class Solution(object):
    def removeKdigits(self, num, k):
        """
        :type num: str
        :type k: int
        :rtype: str
        """
        if len(num) == k:
            return '0'
        stack = []
        for i in num:
            while stack and k and int(stack[-1]) > int(i):
                stack.pop()
                k -= 1
            stack.append(i)
        while k:
            stack.pop()
            k -= 1
        if not stack:
            return '0'
        return str(int(''.join(stack)))