334. Increasing Triplet Subsequence

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

Solution

class Solution {
public:
    bool increasingTriplet(vector<int>& nums) {
        if (nums.size() < 3) return false;
        int first = nums[0], second = INT_MAX;
        for (int i = 1; i < nums.size(); ++i) {
            if (nums[i] > second) return true;
            else if (nums[i] > first) second = min(second, nums[i]);
            else first = min(first, nums[i]);
        }
        return false;
    }
};

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