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, 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.

public class Solution {
    public boolean increasingTriplet(int[] nums) {
        if (nums == null || nums.length < 3) return false;
        
        int m1 = Integer.MAX_VALUE, m2 = Integer.MAX_VALUE;
        for (int a : nums) {
            if (m1 >= a) m1 = a;
            else if (m2 >= a) m2 = a;
            else return true;
        }
        return false;
    }
}

If the ask about k-plet, we can store the m1, m2,..., m_(k-1) to an array and do the same process.