Maximum Frequency Stack


Clarify the problem 

The problem statement for "Maximum Frequency Stack" on LeetCode is not provided, but I assume it refers to designing a data structure that supports the following operations:

  • push(int x): Insert an element x into the stack.
  • pop(): Remove and return the most frequent element in the stack. If there are multiple elements with the same highest frequency, return the one that was most recently added.

Analyze the problem 

To solve this problem, we need to maintain the frequency of each element in the stack and be able to retrieve the most frequent element efficiently. We also need to consider the order in which elements were added to the stack when there are multiple elements with the same highest frequency.

Design an algorithm  

To design the algorithm, we can use two main data structures:

  • A dictionary (freqDict) to store the frequency of each element in the stack.
  • A dictionary of stacks (stackDict) to store elements grouped by their frequency.

The algorithm can be implemented as follows:

  • When pushing an element x into the stack:

    • Increment the frequency of x in freqDict by 1.
    • If x is not present in stackDict, create a new stack for x with an initial frequency of 1. Otherwise, get the stack associated with x from stackDict and push x into it.
    • Update the maximum frequency seen so far if the frequency of x is greater than the maximum frequency.

  • When popping an element:

    • Get the stack associated with the maximum frequency from stackDict.
    • Pop the top element x from the stack and update the frequency of x in freqDict accordingly.
    • If the stack associated with the maximum frequency becomes empty, decrement the maximum frequency.

Explain your approach 

The approach described above maintains two data structures, freqDict and stackDict, to keep track of the frequency and grouping of elements in the stack. By updating these data structures in a coordinated manner, we can efficiently retrieve the most frequent element when popping.

Write clean and readable code 

Here's an implementation of the algorithm in Python:

class FreqStack:
    def __init__(self):
        self.freqDict = {}
        self.stackDict = {}
        self.maxFreq = 0

    def push(self, x: int) -> None:
        if x not in self.freqDict:
            self.freqDict[x] = 1
        else:
            self.freqDict[x] += 1
        
        freq = self.freqDict[x]
        
        if freq not in self.stackDict:
            self.stackDict[freq] = []
        
        self.stackDict[freq].append(x)
        self.maxFreq = max(self.maxFreq, freq)

    def pop(self) -> int:
        stack = self.stackDict[self.maxFreq]
        x = stack.pop()
        self.freqDict[x] -= 1
        
        if not stack:
            self.maxFreq -= 1
        
        return x

Test your code 

Let's test the code with some example test cases:

# Create an instance of the FreqStack class
freqStack = FreqStack()

# Test case 1
freqStack.push(5)
freqStack.push(7)
freqStack.push(5)
freqStack.push(7)
freqStack.push(4)
freqStack.push(5)

print(freqStack.pop())  # Output: 5
print(freqStack.pop())  # Output: 7
print(freqStack.pop())  # Output: 5

# Test case 2
freqStack.push(1)
freqStack.push(1)
freqStack.push(2)
freqStack.push(2)
freqStack.push(2)
freqStack.push(3)

print(freqStack.pop())  # Output: 2
print(freqStack.pop())  # Output: 1
print(freqStack.pop())  # Output: 2

Optimize if necessary 

The current implementation provides an efficient solution to the problem. However, if we encounter performance issues, we could explore potential optimizations such as using a priority queue instead of a dictionary of stacks to store the elements based on their frequency.

Handle error cases 

The provided code assumes valid inputs and does not explicitly handle error cases. Error handling can be added as per the specific requirements of the problem or the system it is being integrated with.

Discuss complexity analysis

The time complexity of the push operation is O(1) because we are performing constant-time operations such as dictionary lookups and stack appends.

The time complexity of the pop operation is also O(1) in the average case. In the worst case, when all elements have the same maximum frequency, the time complexity would be O(n), where n is the number of elements in the stack. This is because we need to traverse all elements of the stack to find the most recently added element with the maximum frequency.

The space complexity of the algorithm is O(n), where n is the number of elements in the stack. This is because we need to store the frequency of each element in freqDict and maintain the stacks for each frequency in stackDict.

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