#9 Medium Time O(n) Space O(n) Animated

Longest Consecutive Sequence

Find the length of the longest consecutive sequence in an unsorted array.

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1 Predict
2 Trace
3 Explain
4 Implement

Pattern Hashing for constant-time memory

Trade one linear scan for a small lookup structure so each value is handled once.

Approach: Hash Set

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Core Insight

Trade one linear scan for a small lookup structure so each value is handled once.

Common Pitfall

Do not overwrite information you still need before checking the current value.

Description

Find the length of the longest consecutive sequence in an unsorted array.

Constraints

Time Complexity: O(n)
Space Complexity: O(n)

Solution — Hash Set

TimeO(n) SpaceO(n)

Implementation Plan

Translate the invariant into these small, testable moves.

Create the lookup structure.
Scan each value once.
Check the needed relationship before updating state.
Return as soon as the answer is determined.

def longestConsecutive(nums):
    numSet = set(nums)
    longest = 0
    
    for num in numSet:
        if (num - 1) not in numSet:
            length = 1
            while (num + length) in numSet:
                length += 1
            longest = max(longest, length)
            
    return longest

Valid Sudoku

Longest Consecutive Sequence

Steps

Description

Constraints

Tags

Solution — Hash Set