O(n log k)-time, O(k)-space alg. for any increasing k-subsequence

  • 1

    Let us consider the general problem, i.e., determining whether there exists an increasing k-subsequence. This problem is essentially equivalent to finding the Longest Increasing Subsequence. The only difference is that we can just return true if an increasing subsequence of length k is found.

    Below is an O(n log k)-time solution using O(k) space. For this particular problem, k = 3, so the runtime is O(n) and the space is O(1).

    // C++ Code
    bool increasingTriplet(vector<int>& nums) {
        vector<int> ans;
        for (int a : nums) {
            if (ans.size() == 0 || a > ans.back()) ans.push_back(a);
            else *lower_bound(ans.begin(), ans.end(), a) = a;
            if (ans.size() >= 3) return true;
        return false;

  • 0

    If I wanted k=0, nums=[] to result in true, how would you do it?

  • 0

    Aha, good catch! I would just add an ugly if statement to handle that. :P

  • 0

    I had that trouble in my own one as well and was like "Ah, no, dammit, I don't want to handle that" :-). But then I found a fairly neat fix. It would even make your code slightly shorter...

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