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, ksuch 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.

class Solution {public:    bool increasingTriplet(vector<int>& nums)     {    if(nums.empty()) return false;    int first=nums[0],second=INT_MAX;    int n=nums.size();    for(int i=1;i<n;i++)    {        if(nums[i]>first&&nums[i]<second) second=nums[i];        else if(nums[i]<first) first=nums[i];        else if(nums[i]>second) return true;    }    return false;    }};