Improved Symmetric Lists

We introduce a new data structure called symlist based on an idea of Tarjan [17]. A symlist is a doubly linked list without any directional information encoded into its cells. In a symlist the two pointers in each cell have no fixed meaning like previous or next in standard lists. Besides the common operations on doubly linked lists, symlists support the reversal of a list and the insertion of a (reversed) symlist into another one, both in constant time. This is an improvement over common implementations, e. g., the Stl class list, where reversal needs linear time. A typical application of symlists is storing the children of a so called Qnode in a PQ-tree, a data structure used in linear time graph planarity testing algorithms. We show that a straightforward implementation of Tarjan’s idea [17] leads to an anomaly when inserting a new element and provide a simple solution to avoid it. Finally, we present an implementation of symlists with iterators that is similar to the Stl class list and its iterators.