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 existsi, j, k
such thatarr[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.

记录两个最小值， 如果有第三个值大于这两个最小值则返回true;