## DEV Community

Abhishek Chaudhary

Posted on

# Maximum Frequency Stack

Design a stack-like data structure to push elements to the stack and pop the most frequent element from the stack.

Implement the `FreqStack` class:

• `FreqStack()` constructs an empty frequency stack.
• `void push(int val)` pushes an integer `val` onto the top of the stack.
• `int pop()` removes and returns the most frequent element in the stack.
• If there is a tie for the most frequent element, the element closest to the stack's top 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
FreqStack freqStack = new FreqStack();
freqStack.push(5); // The stack is [5]
freqStack.push(7); // The stack is [5,7]
freqStack.push(5); // The stack is [5,7,5]
freqStack.push(7); // The stack is [5,7,5,7]
freqStack.push(4); // The stack is [5,7,5,7,4]
freqStack.push(5); // The stack is [5,7,5,7,4,5]
freqStack.pop(); // return 5, as 5 is the most frequent. The stack becomes [5,7,5,7,4].
freqStack.pop(); // return 7, as 5 and 7 is the most frequent, but 7 is closest to the top. The stack becomes [5,7,5,4].
freqStack.pop(); // return 5, as 5 is the most frequent. The stack becomes [5,7,4].
freqStack.pop(); // return 4, as 4, 5 and 7 is the most frequent, but 4 is closest to the top. The stack becomes [5,7].

Constraints:

• `0 <= val <= 109`
• At most `2 * 104` calls will be made to `push` and `pop`.
• It is guaranteed that there will be at least one element in the stack before calling `pop`.

SOLUTION:

``````from collections import Counter

class FreqStack:

def __init__(self):
self.freq = Counter()
self.maxfreq = 0
self.freqgrp = {}

def push(self, val: int) -> None:
newfreq = self.freq[val] + 1
self.freq[val] = newfreq
self.maxfreq = max(self.maxfreq, newfreq)
self.freqgrp[newfreq] = self.freqgrp.get(newfreq, []) + [val]

def pop(self) -> int:
val = self.freqgrp[self.maxfreq].pop()
if len(self.freqgrp[self.maxfreq]) == 0:
self.maxfreq -= 1
self.freq[val] -= 1
return val

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