What's the worst-case time complexity of TreeSet?

Question

It is stated here that

TreeSet has a log(n) time complexity guarantuee for add()/remove()/contains().

But TreeSet uses a binary search tree and in the worst case, binary search trees can have O(n) height. How is the log(n) complexity "guaranteed"?

Answer 1

The implementation rebalances the tree upon inserts.

The O(lg n) time bound for insert is guaranteed by the official documentation . This should not come as a surprise: A classic way to implement sets is as some sort of self-balancing binary search tree .

Java libraries are publicly available, so let's go see for ourselves. TreeSet can be found, eg, here . It is implemented in terms of TreeMap , which is in turn here . As @Don Roby points out, the underlying data structure is a Red-Black tree.