Increasing Triplet Subsequence

Question

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.

Tags

• Array

Thought

Reference: https://discuss.leetcode.com/topic/39807/python-easy-o-n-solution

Code

class Solution(object):
    def increasingTriplet(self, nums):
        """
        :type nums: List[int]
        :rtype: bool
        """
        first, second = float('inf'), float('inf')
        for item in nums:
            if item <= first:
                first = item
            elif item <= second:
                second = item
            else:
                return True
        return False