increasing-triplet-subsequence.js

/**
 * 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
 */

/**
 * @param {number[]} nums
 * @return {boolean}
 */
const increasingTriplet = nums => {
  let n1 = Infinity;
  let n2 = Infinity;

  for (let num of nums) {
    if (num <= n1) {
      n1 = num;
    } else if (num <= n2) {
      n2 = num;
    } else {
      return true;
    }
  }

  return false;
};

export { increasingTriplet };