In problems where you need to find the k-th element in terms of a certain criterion (for example k-th smallest), you could store the numbers in a vector and sort them accordingly using a custom comparator. While this approach allows finding k_th element in O(log N), it does not support updates to the array (removal, insertion, modification ...). The problem with using std::set is iterator advancement has linear complexity. That is why a certain data structure called ordered_set is used. You can learn about them here:https://mirror.codeforces.com/blog/entry/11080. While this data structure may seem like a great alternative to set, this data structure should be used only when necessary as it has a huge constant factor and will result in TLE when used excessively. The problems you provided do not need this data structure, they can be handled with classic binary search with a check() function. If the check function tells you that the current element comes later than the k-th element (k+1th perhaps), you should adjust L or R accordingly. I hope this helps.