Maximum frequency stack [Hash]

Time: O(1); Space: O(N); hard

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].

Notes:

  • 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.

[3]:

import collections

class FreqStack(object):
    """
    Time: O(1) for both push and pop operations.
    Space: O(N), where  is the number of elements in the FreqStack.
    """
    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

[2]:

s = FreqStack()
s.push(5)
s.push(7)
s.push(5)
s.push(7)
s.push(4)
s.push(5)
assert s.pop() == 5
assert s.pop() == 7
assert s.pop() == 5
assert s.pop() == 4