added: longest consecutive sequence

Author: mihirs16

14. Questions/leetcode 128 - longest consecutive sequence.py

@@ -0,0 +1,22 @@
+# longest consecutive sequence | leetcode 128 | https://leetcode.com/problems/longest-consecutive-sequence/
+# set to look-up previous and next numbers; nested while loop is O(2n)
+
+class Solution:
+    def longestConsecutive(self, nums: list[int]) -> int:
+        if nums == []:
+            return 0
+
+        all = set(nums)
+        longest = 0
+
+        for each in all:
+            if each - 1 not in all:
+                curr = each
+                seq = 1
+                while curr + 1 in all:
+                    seq += 1
+                    curr = curr + 1
+                if seq > longest:
+                    longest = seq
+
+        return longest