#334 - Increasing Triplet Subsequence

MediumArray / String

Given an integer array nums, return true if there exists a triple of indices (i, j, k) such that i < j < k and nums[i] < nums[j] < nums[k]. Otherwise return false. Solve in O(n) time and O(1) space.

Input	Output
nums = [1,2,3,4,5]	true
nums = [5,4,3,2,1]	false
nums = [2,1,5,0,4,6]	true

Solution Space

Try it

/**
 * Determines whether any increasing triplet subsequence exists.
 * Maintains the smallest (`first`) and second-smallest (`second`)
 * values seen so far; any value exceeding both completes the triplet.
 *
 * @param nums  Array of integers
 * @returns     true if an increasing triplet subsequence exists
 */
export function increasingTriplet(nums: number[]): boolean {
  let first = Infinity;
  let second = Infinity;

  for (const num of nums) {
    if (num <= first) {
      first = num;
    } else if (num <= second) {
      second = num;
    } else {
      return true; // num > second > first → triplet found
    }
  }

  return false;
}

Approach: greedy two-variable scan – maintain the two smallest values seen so far; if we find a value greater than both, a triplet exists.

Time: O(n) – single pass.

Space: O(1) – only two extra variables.

#238 Product of Array Except Self

#345 Reverse Vowels of a String