Leetcode #2671: Frequency Tracker
In this guide, we solve Leetcode #2671 Frequency Tracker in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Design a data structure that keeps track of the values in it and answers some queries regarding their frequencies. Implement the FrequencyTracker class.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Design, Hash Table
Intuition
Fast membership checks and value lookups are the heart of this problem, which makes a hash map the natural choice.
By storing what we have already seen (or counts/indexes), we can answer the question in one pass without backtracking.
Approach
Scan the input once, using the map to detect when the condition is satisfied and to update state as you go.
This keeps the solution linear while remaining easy to explain in an interview setting.
Steps:
- Initialize a hash map for seen items or counts.
- Iterate through the input, querying/updating the map.
- Return the first valid result or the final computed value.
Example
Input
["FrequencyTracker", "add", "add", "hasFrequency"]
[[], [3], [3], [2]]
Output
[null, null, null, true]
Explanation
FrequencyTracker frequencyTracker = new FrequencyTracker();
frequencyTracker.add(3); // The data structure now contains [3]
frequencyTracker.add(3); // The data structure now contains [3, 3]
frequencyTracker.hasFrequency(2); // Returns true, because 3 occurs twice
Python Solution
class FrequencyTracker:
def __init__(self):
self.cnt = defaultdict(int)
self.freq = defaultdict(int)
def add(self, number: int) -> None:
self.freq[self.cnt[number]] -= 1
self.cnt[number] += 1
self.freq[self.cnt[number]] += 1
def deleteOne(self, number: int) -> None:
if self.cnt[number]:
self.freq[self.cnt[number]] -= 1
self.cnt[number] -= 1
self.freq[self.cnt[number]] += 1
def hasFrequency(self, frequency: int) -> bool:
return self.freq[frequency] > 0
# Your FrequencyTracker object will be instantiated and called as such:
# obj = FrequencyTracker()
# obj.add(number)
# obj.deleteOne(number)
# param_3 = obj.hasFrequency(frequency)
Complexity
The time complexity is O(n). The space complexity is , where is the number of distinct numbers.
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.