Leetcode Problem 334 Increasing Triplet Subsequence.txt

334. Increasing Triplet Subsequence
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.
Note: Your algorithm should run in O(n) time complexity and O(1) space complexity.

Example 1:

Input: [1,2,3,4,5]
Output: true
Example 2:

Input: [5,4,3,2,1]
Output: false


Hint to solve: record the first, second and third number in sequence by linearly traversing the list of numbers. Sneaky algorithm!


class Solution:
    def increasingTriplet(self, nums: List[int]) -> bool:
        
        first = float('inf')
        second = float('inf')
        
        for num in nums:
            if num <= first:
                first = num
            elif num <= second:
                second = num
            else:
                return True
        
        return False