334. Increasing Triplet Subsequence

题目

Given an unsorted array return whether an increasing subsequence of length 3 exists or not in the array.

Formally the function should:

1. Return true if there exists i, j, k such that arr[i] < arr[j] < arr[k] given 0 ≤ i < j < k ≤ n-1
2. 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.

解题思路

见程序注释