Smallest K-Length Subsequence With Occurrences of a Letter — Python

# Time:  O(n)
# Space: O(n)

class Solution(object):
    def smallestSubsequence(self, s, k, letter, repetition):
        """
        :type s: str
        :type k: int
        :type letter: str
        :type repetition: int
        :rtype: str
        """
        stk = []
        suffix = [0]*(len(s)+1)
        for i in reversed(xrange(len(suffix)-1)):
            suffix[i] = suffix[i+1]+(s[i] == letter)
        for i, c in enumerate(s): 
            while stk and stk[-1] > c and len(stk)+(len(s)-i) > k and (stk[-1] != letter or repetition+1 <= suffix[i]):
                repetition += (stk.pop() == letter)
            if len(stk) < min(k-(repetition-(c == letter)), k):
                repetition -= (c == letter)
                stk.append(c)
        return "".join(stk)