Dictionaries and their Implementation: Binary Trees, ...

(辞書とその実装: 二分木など)

Data Structures and Algorithms

8th lecture, November 17, 2016

http://www.sw.it.aoyama.ac.jp/2016/DA/lecture8.html

Martin J. Dürst

© 2009-16 Martin J. Dürst 青山学院大学

Today's Schedule

• Summary of last lecture
• Sorting algorithms faster than O(n log n)
• The dictionary ADT
• Binary trees and their traversal methods
• Binary search tree
• Balanced tree

Summary of Last Lecture

• Quicksort is a very efficiont algorithm for sorting, and a good example to learn about algorithms and their implementation
• In the worst case, quicksort is O(n²); on average, O(n log n)
• Quicksort is a good example for the use of average time complexity and randomized algorithms
• Implementing quicksort requires attention to many details
• Animation of many sorting algorithms: sort.svg
• Sorting based on pairwise comparison is Ω(n log n)

Sorting Faster than O(n log n)

• All sorting algorithms studied so far assume an arbitrary distribution of values
• Decisions are made by comparisons of values;
  the depth of the decision tree is at least O(n log n)
• If there is some knowledge about the value distribution, improvements are possible
• Extreme example: Integers from 1 to n
  →Final place of data can be predicted exactly
  →O(n)
• Radix sort, bin sort

Bin Sort

(also called bucket sort)

Example: Sorting by student number

• Separate data into 10 parts using most significant digit
• Apply recursively to less significant digits
• To manage memory, split separation into two phases
  (one_digit_stable_sort in 8binradix.rb)
  1. Counting number of items in each part
  2. Moving data items
• Complexity is O(n k), where k is the number of digits
• Implementation in Ruby: conceptual_bin_sort in 8binradix.rb

Radix Sort

• Sort once for each digit, starting with the least significant digit
• No need to partition data
• A stable sorting method is necessary
• Complexity is O(n k), where k is the number of digits
• Implementation in Ruby: radix_sort 8binradix.rb

Parallelization of Sorting

• These days, computers do not get faster, but smaller and larger in numbers
• Parallelization becomes important!
• For some tasks, parallelization can be very difficult
• For some tasks, parallelization can be quite easy
• Many sorting algorithms are eazy to parallelize:
  • Bubble sort
  • Merge sort
  • Quick sort

The Dictionary ADT

(caution: Not exactly the same as a (book) dictionary)

• For each data item, there is:
  • A key: Used to identify the data item, e.g. during search
  • A value: All the information besides the key (may be empty)
• Operations
  • Search/find
  • Insert
  • Delete

Simple Dictionary Implementations

• Sorted array: Search is O(log n) (binary search), insertion/deletion is O(n)
• Unordered array/linear list: Search is O(n)
• Ideally, search/insertion/deletion should all be O(log n) or even O(n)
  • Binary search tree (this week)
  • Balanced tree (next week)
  • Hashing (in two weeks)

Binary Tree

• Graph: Consisting of nodes and edges
• Tree: The root does not have any parent; all other nodes have exactly one parent
• Binary tree: Each node has ≦2 children

Traversal Methods for Binary Trees

• Depth first
  • Preorder
  • Inorder
  • Postorder
• Breadth first

Binary Search Tree

(definition/invariants)

• Each node contains one data item
• For any node with key k:
  • All the keys in the left subtree will be <k (or ≦k)
  • All the keys in the right subtree will be >k (or ≧k)
• What to do with multiple identical keys is implementation-dependent

Search in a Search Tree

• Start searching from the root node
• If the search key, compared to the current node
  • Is the same: Return the data item at the current node
  • Is smaller: Search in left subtree
  • Is greater: Search the right subtree
  • Is the empty node: Give up seaching

Insertion into a Search Tree

• Start insertion from the root node
• If the inserted key, compared to the current node
  • Is smaller: Insert item into left subtree
  • Is greater: Insert item into right subtree
  • Is the same: Give up/insert into right subtree, ... (implementation dependent)
• If the current node is empty: Insert item here as a new node (with two empty nodes as children)

Deletion from a Search Tree

• Find the node to delete (using search)
• If the number of (real, non-empty) children is
  • 0: Delete current node (replace with empty node)
  • 1: Replace current node with child
  • 2: Replace current node with smallest child in the right subtree
    (or largest node in left subtree)

Implementation of Search Tree

• Share a single special node (NilNode) everywhere there is no child node
• Pseudocode/implementation in Ruby: 8bintree.rb

Evaluation of Simple Search Tree

• The execution speed depends on the height of the tree
• The best height is O(log n)
• The worst height is O(n)
• The average height is O(log n)
  (assuming that all input orders have the same probability)

Balanced Trees

• In the worst case, the shape of a general search tree is the same as the shape of a linear list
• Because the order of insertions/deletions cannot be changed,
  using randomization (as used for quicksort) to select the dividing item is impossible
• In the case of a complete binary tree, insertion/deletion take too much time

Solution: A tree that is to some degree (but not perfectly) balanced

Top-down 2-3-4 Tree

• The number of children for each node is 2, 3, or 4
• If a node has k children, it stores k-1 keys and data items
  (if the number of children is 2, then this is the same as for a binary search tree)
• The keys stored in a node are the separators for the subtrees
• The tree is of uniform height
• In the lowest layer of the tree, the nodes have no children

Summary

• Bin sort and radix sort are O(n k)
• A dictionary is an ADT storing values that can be found using keys
• A binary search tree is a way to implement a dictionary
• Operations on binary search trees are O(n log n) on average, but O(n) in the worst case
• This problem can be addressed using balanced trees

Homework

(no need to submit)

• Calculate the minimum and maximum height of a binary search tree with n data items
• Using various examples, think about how to insert items into a 2-3-4 tree and propose an algorithm

Glossary

bin sort

ビンソート

most significant digit

最上位の桁

radix sort

基数整列

balanced tree

平衡木

traversal method

辿り方

binary search tree

二分探索木

balanced tree

平衡木

hashing

ハッシュ法

binary tree

二分木

parallelization

並列化

depth first

深さ優先

preorder

行きがけ順

inorder

通りがけ順

postorder

帰りがけ順

breadth first

幅優先

key

キー、鍵

implementation-dependent

実装依存

top-down 2-3-4 tree

トップダウン 2-3-4 木

uniform height

一定の高さ