Maximum Frequency Stack
Problem Description
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 integervalonto 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
Constraints:
- 0 <= val <= 109
- At most
2 * 104calls will be made topushandpop. - It is guaranteed that there will be at least one element in the stack before calling
pop.
Solve it on LeetCode
Approaches
1. Brute-force with Stack
Intuition:
The problem requires us to mimic a stack but with the constraint of popping the most frequent element. If there is a tie in frequency, the most recently pushed element should be popped first.
A basic approach is to maintain a simple stack and a frequency counter dictionary to keep track of the element count. On popping, we traverse the stack from top to bottom to find the element with the maximum frequency.
Code:
- Time Complexity:
push(val): O(1) each time we push an element.pop(): O(n) since we have to traverse the whole stack to find the element with the maximum frequency.- Space Complexity: O(n) where n is the number of elements in the stack.
2. Frequency and Grouped Stacks
Intuition:
To optimize, we can use two hash maps:
frequencyto keep track of the number of occurrences of each element.groupto organize elements by their frequency, where each index ingrouprepresents a stack of elements with that frequency.
This ensures that when we are popping, we can instantly retrieve the most frequent and recent element by looking at the top element of the stack at the current maximum frequency.
Code:
- Time Complexity:
push(val): O(1) since we just increment counters and update stacks.pop(): O(1) for directly accessing and popping from the stack of the maximum frequency.- Space Complexity: O(n) where n is the number of elements in the stack.