https://leetcode.com/problems/increasing-triplet-subsequence/

increasingTripletSequence.js

/**
 * @param {number[]} nums
 * @return {boolean}
 */

// O(n)
// Idea: track the lowest and highest number so far.
// If you find a number higher than the highest so
// far, you've found a triplet sequence!
var increasingTriplet = function(nums) {
    if(nums.length < 3) return false;
    
    let [s, m] = [Infinity, Infinity];
    
    for(let i=0; i<nums.length; i++){
        if(nums[i] > m) return true;
        if(nums[i] <= s) s = nums[i];
        else if(nums[i] <= m) m = nums[i];
    }
    
    return false;
};