maximum-frequency-stack.py

# Time:  O(1)
# Space: O(n)

# Implement FreqStack,
# a class which simulates the operation of a stack-like data structure.
#
# FreqStack has two functions:
#
# push(int x), which pushes an integer x onto the stack.
# pop(), which removes and returns the most frequent element in the stack.
# If there is a tie for most frequent element,
# the element closest to the top of the stack is removed and returned.
#
# Example 1:
#
# Input: 
# ["FreqStack","push","push","push","push","push","push","pop","pop","pop","pop"],
# [[],[5],[7],[5],[7],[4],[5],[],[],[],[]]
# Output: [null,null,null,null,null,null,null,5,7,5,4]
# Explanation:
# After making six .push operations, the stack is [5,7,5,7,4,5] from bottom to top.  Then:
#
# pop() -> returns 5, as 5 is the most frequent.
# The stack becomes [5,7,5,7,4].
#
# pop() -> returns 7, as 5 and 7 is the most frequent, but 7 is closest to the top.
# The stack becomes [5,7,5,4].
#
# pop() -> returns 5.
# The stack becomes [5,7,4].
#
# pop() -> returns 4.
# The stack becomes [5,7].
#
# Note:
# - Calls to FreqStack.push(int x) will be such that 0 <= x <= 10^9.
# - It is guaranteed that FreqStack.pop() won't be called if the stack has zero elements.
# - The total number of FreqStack.push calls will not exceed 10000 in a single test case.
# - The total number of FreqStack.pop calls will not exceed 10000 in a single test case.
# - The total number of FreqStack.push and
#   FreqStack.pop calls will not exceed 150000 across all test cases.

import collections


class FreqStack(object):

    def __init__(self):
        self.__freq = collections.Counter()
        self.__group = collections.defaultdict(list)
        self.__maxfreq = 0

    def push(self, x):
        """
        :type x: int
        :rtype: void
        """
        self.__freq[x] += 1
        if self.__freq[x] > self.__maxfreq:
            self.__maxfreq = self.__freq[x]
        self.__group[self.__freq[x]].append(x)   

    def pop(self):
        """
        :rtype: int
        """
        x = self.__group[self.__maxfreq].pop()
        if not self.__group[self.__maxfreq]:
            self.__group.pop(self.__maxfreq)
            self.__maxfreq -= 1
        self.__freq[x] -= 1
        if not self.__freq[x]:
            self.__freq.pop(x)
        return x


# Your FreqStack object will be instantiated and called as such:
# obj = FreqStack()
# obj.push(x)
# param_2 = obj.pop()