C++ 3sum complexity

Question

I was trying to solve the 3 sum problem in cpp.

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the

Answer 1

You can be in O(n²) with something like:

std::vector> threeSum(std::vector& nums) {
    std::sort(nums.begin(), nums.end());

    std::vector> res;
    for (auto it = nums.begin(); it != nums.end(); ++it) {
        auto left = it + 1;
        auto right = nums.rbegin();
        while (left < right.base()) {
            auto sum = *it + *left + *right;
            if (sum < 0) {
                ++left;   
            } else if (sum > 0) {
                ++right;   
            } else {
                res.push_back({*it, *left, *right});
                std::cout << *it << " " <<  *left << " " << *right << std::endl;
                ++left;
                ++right;
            }
        }
    }
    return res;
}

Demo

I let duplicate handling as exercise.