Posts

Showing posts from 2018

Kotlin Language Features Related to Null Handling

-
Any software engineer with a Java background would find the null handling features in the Kotlin language interesting. Let's summarize this topic with some examples. Nullable types: In Kotlin, types are non-nullable by default. If you want a variable to be able to hold a null value, you need to explicitly declare its type as nullable using the Type? syntax. For example, String? denotes a nullable string, while String represents a non-nullable string. Safe calls (?.): Kotlin introduces the safe call operator (?.) for handling nullable types. It allows you to safely invoke a method or access a property on a nullable object. If the object is null, the expression returns null instead of throwing a NullPointerException. Example: data class Person(val name: String, val age: Int, val address: String?) fun main() {     // Create a person with a nullable address     val person1 = Person("John Doe", 25, "123 Main Street")     val person2 = Person("Jane Doe", 30,
Post a Comment
Read more

A* Search Algorithm and Priority Queues

-
Image
In this blog post, I'll examine the fifteen-puzzle problem . We have an n x n grid filled with numbers from 1 to n-1. Our aim is to reach the position called the "goal board." For example, for a 3x3 grid, our goal board is: 1  2  3 4  5  6 7  8 To reach there, we will be sliding blocks horizontally and vertically to the blank square. The solution approach to this problem utilizes the A* search algorithm . These kinds of algorithms use heuristics to arrive better positions. Let's explain the steps of the algorithm: Create a priority queue and enqueue the initial position. Dequeue the best item using the heuristics. Insert all neighbours of the dequeued item into the queue. Repeat this procedure until the goal board is dequeued.  For each position we need to calculate the heuristic function. For this problem, we can use one of the two priority fuctions: 1) Hamming priority function: The number of blocks in the wrong position. 2) Manhattan prior
Post a Comment
Read more

A Pattern Recognition Problem: Finding Line Segments Effectively In Java

-
Lets say we are given a set of  n  points and we need to find every line segment that connects 4 or more of the points. We have an immutable data type for Point: public class Point implements Comparable {     private final int x;        private final int y;        //...     public int compareTo(Point that) {         if (that.y == y && that.x == x) {             return 0;         }         if (y < that.y) {             return -1;         }         if (y > that.y) {             return 1;         }         if (x < that.x) {             return -1;         }         // x > that.x         return 1;     } } So the comparison first looks at the y coordinate and then there is a tie-break on the x coordinate.      public double slopeTo(Point that) {         if (that.x == x && that.y == y) {             return Double.NEGATIVE_INFINITY;         }         return ((double) (that.y - y)) / (that.x - x);
Post a Comment
Read more

Dynamic Connectivity Problem and the Union-Find Algorithm

-
Lets say we need to connect some objects and query if some objects are connected or not. Below is an example test: @Test     public void testConnectivity() {         MyConnectionDb connectDb = new MyConnectionDb();         connectDb.connect(4, 3);         connectDb.connect(3, 8);         connectDb.connect(6, 5);         connectDb.connect(9, 4);         connectDb.connect(2, 1);         assertTrue(connectDb.isConnected(8, 9));         assertFalse(connectDb.isConnected(5, 4));         connectDb.connect(5, 0);         connectDb.connect(7, 2);         connectDb.connect(6, 1);         connectDb.connect(7, 3);         assertTrue(connectDb.isConnected(5, 3));     } So when some numbers are connected, they form a set and we enlarge that set by applying union operations. And of course we query if given two numbers are connected or not.  I started with a solution using Java collections like HashSet. It is good to get a feel of the problem and c
Post a Comment
Read more

Popular posts from this blog

Trie Data Structure and Finding Patterns in a Collection of Words

-
Image
 I faced a very hard problem recently about finding substrings based on a collection of patterns.  Example For  words = ["Apple", "Melon", "Orange", "Watermelon"]  and  parts = ["a", "mel", "lon", "el", "An"] , the output should be  findSubstrings(words, parts) = ["Apple", "Me[lon]", "Or[a]nge", "Water[mel]on"] . While  "Watermelon"  contains three substrings from the  parts  array,  "a" ,  "mel" , and  "lon" ,  "mel"  is the longest substring that appears first in the string. Note: Number of words and number of parts can be huge! A brute force method or careless usage of Java String methods would make it impossible to handle a large number of words and parts (patterns). Solution Approach Whenever we want to search patterns inside a string, we should think about the Trie data structure:  https://en.wikipedia.
Read more

swapLexOrder: Finding lexicographically largest string

-
The following coding question is really amazing: Given a string  str  and array of  pairs  that indicates which indices in the string can be swapped, return the  lexicographically largest  string that results from doing the allowed swaps. You can swap indices any number of times. Example For  str = "abdc"  and  pairs = [[1, 4], [3, 4]] , the output should be swapLexOrder(str, pairs) = "dbca" . By swapping the given indices, you get the strings:  "cbda" ,  "cbad" ,  "dbac" ,  "dbca" . The lexicographically largest string in this list is  "dbca" . Input/Output [execution time limit] 3 seconds (java) [input] string str A string consisting only of lowercase English letters. Guaranteed constraints: 1 ≤ str.length ≤ 10 4 . [input] array.array.integer pairs An array containing pairs of indices that can be swapped in  str  (1-based). This means that for each  pairs[i] , you can swap elements in  str  that have the indices  p
Read more

A Graph Application in Java: Using WordNet to Find Outcast Words

-
Image
I'll explain our intent with an example: Let's say we have the following words: horse, zebra, cat, bear, table Which one is the outcast? The table.. But how can a computer system find this? We are talking about the whole English language and any word can be in this list. The number of words in the list can be huge, so the solution should use appropriate data structures and algorithms.  We'll use WordNet which is a graph of English words. It contains the semantic relationships between words and our concern is the "is a" relationship. For our example, we know that horse and cat are both animals. "Table" is not an animal and it is the outcast in this situation. But we even have the ability to tell that horse and zebra is very close. Below is a unit test that shows a high level view of our application in Java: public class OutcastTest {     private WordNet wordNet;     private Outcast outcast;     @Before  
Read more