add solution: longest-consecutive-sequence

Author: dusunax

longest-consecutive-sequence/dusunax.py

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+'''
+# Leetcode 128. Longest Consecutive Sequence
+
+keep time complexity O(n) by iterating through the set and accessing elements in O(1) time. ⚖️
+
+## Time and Space Complexity
+```
+TC: O(n)
+SC: O(n)
+```
+
+### TC is O(n):
+- iterating through the set. O(n)
+- accessing elements in the set. O(1)
+- while loop incrementing `current_num` while `current_num + 1 in nums_set`. O(1)
+
+### SC is O(n):
+- creating a set from the list of numbers. O(n)
+'''
+
+class Solution:
+    def longestConsecutive(self, nums: List[int]) -> int:
+        nums_set = set(nums) # O(n)
+        longest_sequence = 0
+
+        for num in nums_set: # O(n)
+            if (num - 1) not in nums_set:
+                current_num = num
+                current_sequence = 1
+            
+                while current_num + 1 in nums_set: # O(1) 
+                    current_num += 1
+                    current_sequence += 1
+
+                longest_sequence = max(current_sequence, longest_sequence)
+
+        return longest_sequence
+