Python solution with detailed explanation

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    Remove K Digits

    General Concept

    • Examples which are interesting: "1234567", "7654321". The first is strictly increasing and the second is strictly decreasing.
    • How do you choose to remove the first element?
    • You will scan from left to right and remove the first peak element. Example 1 that would be 7, the last element. Example 2 that would be 7, the first
    • Why the first peak element? You want to remove the first largest element from left which has a smaller element on right, so that the smaller element can fill in for the large element that we removed.

    Brute Force

    Stack Simulation

    • Now how can we do the above efficiently? We can use a stack to simulate this.
    • if x > st[-1], push x. If x < st[-1], pop all elements from the stack which are greater than x while making sure we do not pop more than a total of k elements.
    • Adjust for boundary conditions.
    class Solution(object):
        def removeKdigits(self, num, k):
            :type num: str
            :type k: int
            :rtype: str
            st, required = [], k
            for x in num:
                while required and st and st[-1] > x:
                    required = required-1
            result = "".join(st[0:len(num)-k]).lstrip("0")
            return result if len(result) else "0"

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