LeetCode 402. Remove K Digits 栈

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.

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12219 should be 1219 instead of 1221

• Sequential Alphabet Order! Stack is one option
• "0" is a potential bug

Right codes:

class Solution:
    def removeKdigits(self, num: str, k: int) -> str:
        res = []
        for ch in num:
            while (k > 0 and res and ch < res[-1]):
                res.pop()
                k -= 1
            if (res or ch != "0"):
                res.append(ch)
        for i in range(k):
            if (res):
                res.pop()
        return ''.join(res) if res else "0"