MEDIUM NC#42 Linked List / Floyd's Algorithm
287. Find the Duplicate Number
📖 Problem
Given an array of integers nums containing n + 1 integers where each integer is in the range [1, n] inclusive. There is only one repeated number in nums, return this repeated number. You must solve the problem without modifying the array nums and using only constant extra space.
🧠 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
- → Treat array as linked list where nums[i] points to nums[nums[i]]
- → This creates a cycle because of duplicate
- → Use Floyd's cycle detection (fast/slow pointers)
- → When cycle found, find entrance (the duplicate)
- → Time: O(n), Space: O(1)
🛠️ Hints & Pitfalls
Hints
- •Treat array as linked list where nums[i] points to nums[nums[i]]
- •This creates a cycle because of duplicate
- •Use Floyd's cycle detection (fast/slow pointers)
Common Pitfalls
- •When cycle found, find entrance (the duplicate)
- •Time: O(n), Space: O(1)
🧪 Test Cases
Hidden tests on submit: 1
Test Case 1
Not run Input:
findDuplicate([1,3,4,2,2]); Expected:
2 Test Case 2
Not run Input:
findDuplicate([3,1,3,4,2]); Expected:
3 Test Case 3
Not run Input:
findDuplicate([3,3,3,3,3]); Expected:
3 📝 Code Editor
📤 Output