334. Increasing Triplet Subsequence.cpp

/*
Problem Description:
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 ¡Ü i < j < k ¡Ü 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.size()<3) return false;
        int a=INT_MAX,b=INT_MAX;
        for(auto num:nums)
        {
            if(num<=a)
            {
                a=num;
            }
            else
            {
                if(num<=b)
                {
                    b=num;
                }
                else
                {
                    return true;
                }
            }
        }
        return false;
    }
};