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.

Note: Your algorithm should run in O(n) time complexity and O(1) space complexity.

Example 1:

Input: [1,2,3,4,5]
Output: true

Example 2:

Input: [5,4,3,2,1]
Output: false

bool increasingTriplet(vector<int>& nums) { // time: O(n); space: O(1)
    int c1 = INT_MAX, c2 = INT_MAX;
    for (int num : nums) {
        if (num <= c1) c1 = num;
        else if (num <= c2) c2 = num;
        else return true;
    }
    return false;
}