895. Maximum Frequency Stack

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 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.

 

Solutions

Solution: Hash Map

• Time complexity: O(1)
• Space complexity: O(n)

 

JavaScript

const FreqStack = function () {
  this.maxFrequency = 0;
  this.frequencyMap = new Map();
  this.frequencies = [];
};

/**
 * @param {number} val
 * @return {void}
 */
FreqStack.prototype.push = function (val) {
  const frequency = (this.frequencyMap.get(val) ?? 0) + 1;

  this.frequencyMap.set(val, frequency);
  if (!this.frequencies[frequency]) this.frequencies[frequency] = [];
  this.frequencies[frequency].push(val);
  this.maxFrequency = Math.max(frequency, this.maxFrequency);
};

/**
 * @return {number}
 */
FreqStack.prototype.pop = function () {
  const frequencies = this.frequencies[this.maxFrequency];
  const value = frequencies.pop();
  const frequency = this.frequencyMap.get(value);

  this.frequencyMap.set(value, frequency - 1);
  if (!frequencies.length) this.maxFrequency -= 1;
  return value;
};

/**
 * Your FreqStack object will be instantiated and called as such:
 * var obj = new FreqStack()
 * obj.push(val)
 * var param_2 = obj.pop()
 */