402 Remove K Digits.py

"""
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.
Author: Rajeev Ranjan

"""


class Solution(object):
    def removeKdigits(self, num, k):
        """
        Stack and greedy.
        The leading digits should be as small as possible
        :type num: str
        :type k: int
        :rtype: str
        """
        stk = []  # result after removal
        for char in num:
            while k and stk and stk[-1] > char:
                stk.pop()
                k -= 1

            stk.append(char)

        for _ in xrange(k): stk.pop()

        return ''.join(stk).lstrip('0') or '0'