LeetCode Q334: 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.

Your algorithm should run in O(n) time complexity and O(1) space complexity.

Examples:
Given [1, 2, 3, 4, 5],
return true.

Given [5, 4, 3, 2, 1],
return false.

Solution:

The solution of the question is same to Q300: Longest Increasing Subsequence.

Time complexity O(nlogn)

	class Solution {
	public:
	bool increasingTriplet(vector<int>& nums) {
	if(nums.empty())
	return 0;
	set<int> set;
	for(int i=0; i<nums.size(); i++){
	auto it = set.lower_bound(nums[i]);
	if(it==set.end()){
	set.insert(nums[i]);
	if(set.size()>=3)
	return true;
	}else{
	set.erase(it);
	set.insert(nums[i]);
	}
	}
	return false;
	}
	};

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