**Self-Balancing Binary Search Trees** are * height-balanced* binary search trees that automatically keeps height as small as possible when insertion and deletion operations are performed on tree. The height is typically maintained in order of Log n so that all operations take O(Log n) time on average.

**Examples :**

Red Black Tree

AVL Tree:

**Language Implementations : **set and map in C++ STL. TreeSet and TreeMap in Java. Most of the library implementations use Red Black Tree. Python standard library does not support Self Balancing BST. In Python, we can use bisect module to keep a set of sorted data. We can also use PyPi modules like rbtree (implementation of red black tree) and pyavl (implementation of AVL tree).

**How do Self-Balancing-Tree maintain height?**

A typical operation done by trees is rotation. Following are two basic operations that can be performed to re-balance a BST without violating the BST property (keys(left) < key(root) < keys(right)).
1) Left Rotation
2) Right Rotation

T1, T2 and T3 are subtrees of the tree rooted with y (on the left side) or x (on the right side) y x / \ Right Rotation / \ x T3 – – – – – – – > T1 y / \ < - - - - - - - / \ T1 T2 Left Rotation T2 T3 Keys in both of the above trees follow the following order keys(T1) < key(x) < keys(T2) < key(y) < keys(T3) So BST property is not violated anywhere.

We have already discussed AVL tree, Red Black Tree and Splay Tree. In this acrticle, we will compare the efficiency of these trees:

Metric | RB Tree | AVL Tree | Splay Tree |
---|---|---|---|

Insertion inworst case | O(1) | O(logn) | Amortized O(logn) |

Maximum heightof tree | 2*log(n) | 1.44*log(n) | O(n) |

Search inworst case | O(logn),Moderate | O(logn),Faster | Amortized O(logn),Slower |

Efficient Implementation requires | Three pointers with color bit per node | Two pointers with balance factor pernode | Only two pointers withno extra information |

Deletion inworst case | O(logn) | O(logn) | Amortized O(logn) |

Mostly used | As universal data structure | When frequent lookups are required | When same element isretrieved again and again |

Real world Application | Multiset, Multimap, Map, Set, etc. | Database Transactions | Cache implementation, Garbage collection Algorithms |

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