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hardDSA

How do you find the top K frequent elements?

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01

Understand the problem

Explain top-K.

heaptop-k
02

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03

Study the solution

Count frequencies in a hash map, then keep a min-heap of size K by frequency (O(n log k)), or use bucket sort by frequency for O(n). Quickselect on frequencies is another average-O(n) approach.

Solution ready — 2 min read

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04

Read the code

Heap + bucket sort
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from collections import Counter
import heapq

def top_k_frequent(nums, k):
    counts = Counter(nums)
    # O(n log k) with a heap:
    return heapq.nlargest(k, counts.keys(), key=counts.get)

def top_k_bucket(nums, k):
    counts = Counter(nums)
    buckets = [[] for _ in range(len(nums) + 1)]
    for val, freq in counts.items():
        buckets[freq].append(val)
    result = []
    for freq in range(len(buckets) - 1, 0, -1):
        for val in buckets[freq]:
            result.append(val)
            if len(result) == k:
                return result
    return result


# --- demo ---
print(top_k_frequent([1, 1, 1, 2, 2, 3], 2))   # [1, 2]
print(top_k_bucket([1, 1, 1, 2, 2, 3], 2))     # [1, 2]
05

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