Hash Sets & Their Patterns

Membership testing in O(1), eliminating duplicates and tracking visited nodes

A set stores unique elements and provides O(1) average-case:

add(x), add element

x in s, membership test

remove(x), remove element

Sets are the fastest way to answer "have I seen this before?"

Longest consecutive sequence, O(n)python

def longest_consecutive(nums):
    num_set = set(nums)  # O(1) lookups
    longest = 0

    for n in num_set:
        # Only start counting from the beginning of a sequence
        if n - 1 not in num_set:
            current = n
            streak = 1
            while current + 1 in num_set:
                current += 1
                streak += 1
            longest = max(longest, streak)

    return longest

print(longest_consecutive([100, 4, 200, 1, 3, 2]))  # → 4 (1,2,3,4)

🤔Quick Check

You're doing BFS on a graph. Why use a "visited" set instead of a list?

🎯

Phase Complete!

Hash Maps & Sets

Hash maps and sets are some of the most powerful tools in your arsenal. You'll use them in almost every medium/hard interview problem. Now for trees!

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Practice Exercises

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Exercise 1 of 1easy

⏱ 00:00

Happy Number

A happy number: repeatedly sum the squares of its digits. If you reach 1, it's happy. If you enter a cycle, it's not.
Expected output: True

solution.py

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