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Showing posts from January, 2019

The WeakReference class, monitoring memory leak and garbage collection in a Java application

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 Below is a Stack implementation that uses an internal resizeable array structure.  public class MyStack< T > implements Stack< T > { private static final int CAPACITY = 100 ; private Object[] array ; private int pos = 0 ; public MyStack () { this . array = new Object[ CAPACITY ] ; } @Override public void push ( T item) { if ( pos >= array . length / 2 ) { Object[] newArray = new Object[ pos * 2 ] ; System. arraycopy ( array , 0 , newArray , 0 , array . length ) ; array = newArray ; } array [ pos ++] = item ; } @Override public T pop () { if (isEmpty()) { throw new RuntimeException( "empty stack" ) ; } @SuppressWarnings ( "unchecked" ) T item = ( T ) array [ pos - 1 ] ; pos -= 1 ; return item ; } @Override @SuppressWarnings ( "unchecked" ) public T peek...
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Key-indexed counting for String manipulation in Java

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We know that the fastest sorting algorithms that is based on comparing the elements (compareTo method) have a complexity of O(n logn) But, is there a faster way for certain conditions? Yes. Let's say we have an array of 11 letters with values from a to f {e, e, f, a, b, a, a, c, d, a, a} To sort these letters, we'll create an array that stores the frequencies of the letters. The index will be the letter itself. For example, for letter a, the index will be 0. For letter c, the index will be 2 and so on.. Frequency array for {a, b, c, d, e, f} is {5, 1, 1, 1, 2, 1} The second step is calculating the cumulative counts which is {5, 6, 7, 8, 10, 11} We need to use this cumulative counts to learn the position of each letter. We know that the position of letter a will be zero. So the array will start with 0 as the first position for a: indexes:            {a, b, c, d, e, f , - } cumulative array:   {0, 5, 6, 7, 8, 10, 11} This cumulati...
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Geometric Objects and KdTrees

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Lets assume we have a set of points and we want to find all of the points that are contained in a query rectangle. And we also want to find the closest point to a query point. These are called "range search" and "nearest-neighbour search". I'll show the usage of a 2d-tree for this problem. Kd-trees have many applications including computer animation and image retrieval. Lets start with a brute-force approach and look at all the points in the input without any efficient data structure for the specific problem: public class PointSET {     private TreeSet set = new TreeSet<>();     public void insert(Point2D point) {         if (point == null) {             throw new IllegalArgumentException();         }         set.add(point);     }     public boolean contains(Point2D point) {          if (point == ...
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Trie Data Structure and Finding Patterns in a Collection of Words

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 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....
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My Crappy Looking Solution to "Binary Tree Common Ancestor" Problem

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I was able to solve this problem and make all the tests pass: https://www.hackerrank.com/challenges/binary-search-tree-lowest-common-ancestor/problem But I'm not happy with the code, so later I need to refactor the code. Here it is: static Node lca(Node root, int v1, int v2) { ArrayList result = new ArrayList<>(); Node result2 = traverse (root, v1, v2, result); if (result.isEmpty()) return result2; return result.get( 0 ); } private static Node traverse(Node root, int v1, int v2, ArrayList result) { if (!result.isEmpty()) { return result.get( 0 ); } if (root. left == null && root. right == null ) { return root; } if (root. data == v1 || root. data == v2) { return root; } Node node1 = null , node2 = null ; if (root. left != null ) node1 = traverse (root. left , v1, v2, result); if (root. right != null ) node2 = traverse (root. right , v1, v...
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A Graph Application in Java: Using WordNet to Find Outcast Words

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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 ...
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