MEDIUM NC#9 Blind #31 Arrays & Hashing
128. Longest Consecutive Sequence
📖 Problem
Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence. You must write an algorithm that runs in O(n) time.
🧠 Visual Learning Aid
1 Model the input into the right structure
2 Choose the core technique and invariant
3 Execute step-by-step with a sample
4 Validate complexity and edge cases
JS/TS Refreshers
- •Array methods (`push`, `pop`, `shift`, `slice`)
- •Object/Map/Set usage patterns
- •Function parameter and return typing
Logical Thinking Concepts
- •Define invariants before coding
- •Check edge cases first (`[]`, single element, duplicates)
- •Estimate time/space before implementation
💡 Approach
- → Use set for O(1) lookups
- → Only start counting from beginning of sequences
- → A number is a sequence start if num-1 is not in set
- → Time: O(n), Space: O(n)
🧭 Prerequisites
🛠️ Hints & Pitfalls
Hints
- •Use set for O(1) lookups
- •Only start counting from beginning of sequences
- •A number is a sequence start if num-1 is not in set
Common Pitfalls
- •Time: O(n), Space: O(n)
🧪 Test Cases
Hidden tests on submit: 2
Test Case 1
Not run Input:
longestConsecutive([100,4,200,1,3,2]); Expected:
4 Test Case 2
Not run Input:
longestConsecutive([0,3,7,2,5,8,4,6,0,1]); Expected:
9 Test Case 3
Not run Input:
longestConsecutive([9,1,4,7,3,-1,0,5,8,-1,6]); Expected:
7 📝 Code Editor
📤 Output