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.

解题思路：遍历数组，遍历过程中更新已经遍历的数组元素中的最小值和次小值，如果后续有元素比这两个值都大，说明存在这样的一个递增子序列。

class Solution(object):

    def increasingTriplet(self, nums):

        """

        :type nums: List[int]

        :rtype: bool

        """

        min =

        smin =

        for i in nums:

            if i < min:

                min = i

            elif i<smin:

                smin = i

            else:

                return True

        return False