Increasing triplet subsequence in O(n) time and O(1) space

Author: BarthJr

increasing_triplet_subsequence.py

@@ -0,0 +1,38 @@
+# Given an unsorted array return whether an increasing subsequence of length 3 exists or not in the array.
+#
+# Formally the function should:
+#
+# Return true if there exists i, j, k
+# such that arr[i] < arr[j] < arr[k] given 0 ≤ i < j < k ≤ n-1 else return false.
+"""
+>>> increasing_triplet([1, 2, 3, 4, 5])
+True
+>>> increasing_triplet([5, 4, 3, 2, 1])
+False
+>>> increasing_triplet([5, 4, 3])
+False
+>>> increasing_triplet([1, 2, 3])
+True
+>>> increasing_triplet([3, 2, 4, 1, 5])
+True
+>>> increasing_triplet([])
+False
+>>> increasing_triplet([1, 1, 1])
+False
+>>> increasing_triplet([5, 1, 5, 5, 2, 5, 4])
+True
+
+"""
+from typing import List
+
+
+def increasing_triplet(nums: List[int]) -> bool:
+    i = j = k = float('inf')
+    for num in nums:
+        if num <= i:
+            i = num
+        elif num <= j:
+            j = num
+        elif num <= k:
+            return True
+    return False