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Longest Consecutive Sequence

Certification: Advanced Level Accuracy: 0% Submissions: 0 Points: 20

Write a C program to find the length of the longest consecutive elements sequence in an unsorted array of integers. The algorithm must run in O(n) time complexity. A consecutive sequence is a sequence of integers where each number is exactly one more than the previous number.

Example 1
    
      
  • Input: nums = [100, 4, 200, 1, 3, 2]
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  • Output: 4
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  • Explanation: 
    • The array contains [100, 4, 200, 1, 3, 2]. 
    • The longest consecutive sequence is [1, 2, 3, 4]. 
    • The length of this sequence is 4.
    •
Example 2
    
      
  • Input: nums = [0, 3, 7, 2, 5, 8, 4, 6, 0, 1]
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  • Output: 9
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  • Explanation: 
    • The array contains [0, 3, 7, 2, 5, 8, 4, 6, 0, 1]. 
    • The longest consecutive sequence is [0, 1, 2, 3, 4, 5, 6, 7, 8]. 
    • The length of this sequence is 9.
    •
Constraints
    
      
  • 0 ≤ nums.length ≤ 10^5
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  • -10^9 ≤ nums[i] ≤ 10^9
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  • Time Complexity: O(n)
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  • Space Complexity: O(n)
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  • Cannot use sorting algorithms
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Hint
Solution

Solution Hints

    
      
  • Use a hash set to store all numbers for O(1) lookup time
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  • For each number, check if it's the start of a sequence (no predecessor exists)
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  • If it's a sequence start, count consecutive numbers from that point
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  • Keep track of the maximum sequence length encountered
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  • Skip numbers that are not sequence starts to avoid redundant work
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Steps to solve by this approach:

 Step 1: Create a hash set and add all numbers from the array for O(1) lookup operations.

 Step 2: Iterate through each number in the original array to find potential sequence starts.

 Step 3: For each number, check if it's the beginning of a sequence (predecessor doesn't exist).

 Step 4: If it's a sequence start, count consecutive numbers by incrementing and checking existence.
 Step 5: Keep track of the maximum sequence length found during the iteration.
 Step 6: Skip numbers that are not sequence starts to avoid redundant counting.

 Step 7: Return the maximum consecutive sequence length after checking all potential starts.

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