cppreference.com > C++ Lists > merge merge Syntax:
      #include <list>
      void merge( list &lst );
      void merge( list &lst, BinPred compfunction );
The function merge() merges the list with lst, producing a combined list that is ordered with respect to the < operator. If compfunction is specified, then it is used as the comparison function for the lists instead of <. merge() runs in linear time. Related topics: Container operators insert splice