DATA_STRUCTS.md

Data structures

String

substring

String s = "0123456789";
// Returns a new string which is the substring of a specified string
String sub = s.substring(1,4)
// sub -> "123" (1-inclusive, 4-exclusive)
sub = s.substring(6)
//sub -> "6789"

compare

compareTo returns:

• An int value: 0 if the string is equal to the other string.
• < 0 if the string is lexicographically less than the other string
• > 0 if the string is lexicographically greater than the other string (more characters)

s1.compareTo(s2);
s1.compareToIgnoreCase(s2);

equals

s1.equals(s2);
s1.equalsIgnoreCase(s2);

Array of chars

char[] chars = s1.toCharArray();

locating chars/sequences

// Returns the position of the last found occurrence of 
// specified characters in a string
lastIndexOf()
// Returns the position of the first found occurrence of 
// specified characters in a string
indexOf(int char, int fromIndex)
indexOf(String str

replace

// returns A new String, where the specified character
// has been replaced by the new character(s)
String myStr = "Hello";
System.out.println(myStr.replace('l', 'p'));

//Replaces the first occurrence of a substring that matches the
// given regular expression with the given replacement
replaceFirst()
        
//Replaces each substring of this string that matches the given regular expression 
// with the given replacement
replaceAll()
String replaceString=s1.replaceAll("a","e");//replaces all occurrences of "a" to "e"  

split

String s = "Welcome to Baeldung";
String[] expected1 = new String[] { "Welcome", "to", "Baeldung" };
String[] expected2 = new String[] { "Welcome", "to Baeldung" };
    
assertArrayEquals(expected1, s.split(" "));
assertArrayEquals(expected2, s.split(" ", 2));

Array

(Arrays class)

to list

String a[] = new String[] { "A", "B", "C", "D" };
// Getting the list view of Array
List<String> list = Arrays.asList(a);

fill

int ar[] = {2, 2, 1, 8, 3, 2, 2, 4, 2};
// To fill complete array with a particular value
Arrays.fill(ar, 10);

sort

int numbers[] = {2, 2, 1, 8, 3, 2, 2, 4, 2};

// Using Arrays.parallelSort()
Arrays.parallelSort(numbers);
//or 
Arrays.sort(numbers)

Lists

modify an element

cars.set(0, "Opel");

remove an element

cars.remove(0);

clear a list

cars.clear();

LinkedList

Stack

initialise

Stack<E> stack = new Stack<E>();

push

stack.push(x);

pop

Integer y = stack.pop();

peek

Displaying element on the top of the stack

Integer element = (Integer) stack.peek();

search

Integer pos = (Integer) stack.search(element);
  
if(pos == -1)
    System.out.println("Element not found");
else
    System.out.println("Element is found at position: " + pos);

Queue

initialise

PriorityQueue<Integer> pq = new PriorityQueue<>();

add

pq.add(1);

get first

System.out.println("head: "+pq.peek());

iterate

Iterator itr=pq.iterator();
while(itr.hasNext()){
    System.out.println(itr.next());
}

poll

kinda like pop but from the other side

System.out.println("queue poll: " + pq.poll());

Hash map

initialise and add items

HashMap<String, String> capitalCities = new HashMap<String, String>();

// Add keys and values (Country, City)
capitalCities.put("England", "London");
capitalCities.put("Germany", "Berlin");
capitalCities.put("Norway", "Oslo");
capitalCities.put("USA", "Washington DC");
System.out.println(capitalCities);

access an item

capitalCities.get("England");

remove an item

capitalCities.remove("England");

clear

capitalCities.clear();

iterate

for (String i : people.keySet()) {
    System.out.println("key: " + i + " value: " + people.get(i));
}

contains?

// returns true or false
map.containsKey(4);
map.containsValue("aaaa");

modify a value

map.put(key, map.get(key) + 1);

Hash set

A HashSet is a collection of items where every item is unique

public class Main {
  public static void main(String[] args) {
    HashSet<String> cars = new HashSet<String>();
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    cars.add("BMW");
    cars.add("Mazda");
    System.out.println(cars);
  }
}

item exists?

cars.contains("Mazda");

remove

cars.remove("Volvo");