334. Increasing Triplet Subsequence.py

# 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.
# Your algorithm should run in O(n) time complexity and O(1) space complexity.

# Examples:
# Given [1, 2, 3, 4, 5],
# return true.

# Given [5, 4, 3, 2, 1],
# return false.

# Credits:
# Special thanks to @DjangoUnchained for adding this problem and creating all test cases.
# Two Pointer
class Solution(object):
    def increasingTriplet(self, nums):
        """
        :type nums: List[int]
        :rtype: bool
        """
        first = second = float('inf')
        for n in nums:
            if n <= first:
                first = n
            elif n <= second:
                second = n
            else:
                return True
        return False