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Longest Consecutive Sequence

Certification: Advanced Level Accuracy: 0% Submissions: 0 Points: 15

Write a C# program to implement the LongestConsecutive(int[] nums) function, which finds the length of the longest consecutive elements sequence in an unsorted array of integers. Your function should run in O(n) time complexity.

Example 1
    
  
  • Input: nums = [100, 4, 200, 1, 3, 2]
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  • Output: 4
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  • Explanation: 
    
      
      
    • The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.
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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 longest consecutive elements sequence is [0, 1, 2, 3, 4, 5, 6, 7, 8]. Therefore its length is 9.
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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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Hint
Solution

Solution Hints

    
  
  • Use a HashSet to efficiently check if elements exist in the array
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  • For each number, check if it's the start of a sequence by verifying if its predecessor exists
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  • If it's a start, count the consecutive sequence length
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  • Keep track of the maximum sequence length encountered
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  • A single pass through the array is sufficient for O(n) time complexity
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Steps to solve by this approach:

 Step 1: Create a HashSet to store all numbers from the input array for O(1) lookups.
 Step 2: Initialize a variable to track the maximum length of consecutive sequence.
 Step 3: Iterate through each number in the array.
 Step 4: For each number, check if it's the start of a sequence by verifying its predecessor doesn't exist.
 Step 5: If it's a start of a sequence, count how long the sequence extends by checking for consecutive numbers.
 Step 6: Update the maximum length if the current sequence is longer.
 Step 7: Return the maximum length found after checking all possible sequences.

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