334. Increasing Triplet Subsequence

• Difficulty: Medium

• Topics:

• Similar Questions:

  • Longest Increasing Subsequence

Problem:

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

Solutions:

class Solution {
public:
    bool increasingTriplet(vector<int>& nums) {
        int smallest = INT_MAX;
        int secondSmallest = INT_MAX;

        for (auto num : nums) {
            if (num <= smallest) {
                smallest = num; // it is important that secondSmallest is not updated!
            } else if (num <= secondSmallest) {
                secondSmallest = num;
            } else {
                return true;
            }
        }

        return false;
    }
};