We use a Hash Set to store all numbers. For each number, we only start counting the length of the sequence if it is the "start" of a sequence (i.e., num - 1 is not in the set). This ensures each element is visited only twice.

1. Create a set num_set from the array nums.
2. Initialize longest_streak = 0.
3. Iterate through each num in the num_set:
   • Check if num - 1 is not in num_set.
   • If it's not, it's the start of a sequence.
   • Set current_num = num and current_streak = 1.
   • While current_num + 1 is in num_set, increment current_num and current_streak.
   • Update longest_streak = max(longest_streak, current_streak).
4. Return longest_streak.

• Time Complexity: O(N), because each number is processed exactly twice (once when building the set and once inside the loop).
• Space Complexity: O(N), for the hash set.

def longest_consecutive(nums):
    num_set = set(nums)
    longest_streak = 0
    
    for num in num_set:
        # Only start if it is the start of a sequence
        if num - 1 not in num_set:
            current_num = num
            current_streak = 1
            
            while current_num + 1 in num_set:
                current_num += 1
                current_streak += 1
                
            longest_streak = max(longest_streak, current_streak)
            
    return longest_streak