if you have 4 circles such that all pairs intersect with each other then here's how the algorithm will work:

the sweeping line will move until it touches the first circle from the left, you will insert its Y coordinates to std::set now the line will continue sweeping until it touches another circle from left and you will add its Y coordinates to std::set, since there was only one circle in the std::set the newly added circle will come either after or before it so you have to check if those two circles intersect and they indeed intersect because all circles intersect with each other so you just return that there's intersection and terminate the algorithm.