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Longest Consecutive Sequence

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

Write a C++ program to find the length of the longest consecutive elements sequence in an unsorted array of integers. A consecutive sequence is a sequence of integers where each value is exactly one more than the previous value.

Example 1
    

  • Input: nums = [100,4,200,1,3,2]
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  • Output: 4
    • 

  • Explanation: 
      

    • Step 1: Store all numbers in a hash set for O(1) lookups.
      • 

    • Step 2: For each number, check if it's the start of a sequence (no number-1 exists in the set).
      • 

    • Step 3: If it's a sequence start, count consecutive numbers (number+1, number+2, etc.).
      • 

    • Step 4: The longest sequence is [1,2,3,4] with length 4.
      • 

    •
Example 2
    

  • Input: nums = [0,3,7,2,5,8,4,6,0,1]
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  • Output: 9
    • 

  • Explanation: 
      

    • Step 1: Store all numbers in a hash set for O(1) lookups.
      • 

    • Step 2: For each number, check if it's the start of a sequence (no number-1 exists in the set).
      • 

    • Step 3: If it's a sequence start, count consecutive numbers (number+1, number+2, etc.).
      • 

    • Step 4: The longest sequence is [0,1,2,3,4,5,6,7,8] with length 9.
      • 

    •
Constraints
    

  • 0 ≤ nums.length ≤ 10^5
    • 

  • -10^9 ≤ nums[i] ≤ 10^9
    • 

  • Your algorithm should run in O(n) time complexity
    • 

  • Time Complexity: O(n) where n is the length of the array
    • 

  • Space Complexity: O(n)
    •
Hash MapSetTech MahindraArctwist
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Hint
Solution

Solution Hints

    
  
  • Use a hash set to enable O(1) lookups
    • 
  
  • Only start checking sequences from the smallest element of each sequence
    • 
  
  • For each number, check if it's the start of a sequence by looking for num-1
    • 
  
  • If it's the start of a sequence, count how long the sequence is
    • 
  
  • Consider edge cases like empty arrays or arrays with duplicate elements
    •

Steps to solve by this approach:

 Step 1: Insert all array elements into a hash set for O(1) lookups.
 Step 2: Iterate through each number in the hash set.
 Step 3: For each number, check if it's the start of a sequence by confirming num-1 doesn't exist.
 Step 4: If it's the start of a sequence, count consecutive elements by checking for num+1, num+2, etc.
 Step 5: Update the longest streak length whenever a longer sequence is found.
 Step 6: Handle edge cases like empty arrays.
 Step 7: Return the maximum consecutive sequence length found.

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