APUNTS_JAVA.md

Apunts Java - LeetCode Problemes

HashMap

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

Add an Item

map.add(key, value)

Access an Item

value = map.get()

Remove an Item

map.remove(key)

Get Keys

for (String i : map.keySet()) {
  System.out.println(i);
}

Get Values

for (String i : map.values()) {
  System.out.println(i);
}

Keys to ArrayList

ArrayList<String> listOfKeys = map.keySet().stream().collect(Collectors.toCollection(ArrayList::new));

new ArrayList(map.keySet())

Values to ArrayList

ArrayList<Integer> listOfValues = map.values().stream().collect(Collectors.toCollection(ArrayList::new));

new ArrayList(map.values())

HashSet

Set<Integer> set = new HashSet<Integer>();

Iterar en HashSet

Iterator values = set.iterator();
while (values.hasNext()) {
    System.out.println((int) values.next());
}

Eliminar números repetits:

Integer[] wrapper = new Integer[nums.length];
...
Set<Integer> set = new HashSet<Integer>(Arrays.asList(wrapper));

Arrays

Ordenar array estàtic

Arrays.sort(nums);

Clonar array estàtic

int[] copiedArray = nums.clone();

int[] to Integer[]

Integer[] list = Arrays.stream( data ).boxed().toArray(Integer[]::new);

int[] to ArrayList

List<Integer> list = Arrays.stream( data ).boxed().collect(Collectors.toList());

Strings

String to Integer

String.valueOf(n)

Integer to String

Integer.toString(i)

Characters

Lower Case

char c1 = Character.toLowerCase(c);

Letter Or Digit

Character.isLetterOrDigit(c)

Queue

Queue<Integer> queue = new LinkedList<Integer>();

Add element

Inserts the specified element into this queue.

queue.add(e)

Poll head

Retrieves and removes the head of this queue, or returns null if this queue is empty.

queue.poll()

Remove head

Retrieves and removes the head of this queue.

queue.remove()

Peek head

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

queue.peek()

Stack

Stack<Integer> stack = new Stack<Integer>()

Empty Stack

Tests if this stack is empty.

empty()

Peek Stack

Looks at the object at the top of this stack without removing it from the stack.

peek()

Pop Stack

Removes the object at the top of this stack and returns that object as the value of this function.

pop()

Push Element

Pushes an item onto the top of this stack.

push(e)

Fibonacci

Algorisme Iteratiu

public int Fibonacci(int n) {
    int sum = 1;
    int before = 1;
    int aux = 0;

    for (int i = 2; i <= n; i++) {
        aux = sum;
        sum += before;
        before = aux;
    }
    return sum;
}   

Algorisme Recursiu

public int Fibonacci(int n) {
    if (n == 0) {
      return 0;
    } else if (n < 3) {
      return 1;
    } else {
      return Fibonacci(n-1) + Fibonacci(n-2);
    }
}

Random Numbers

Random randomGenerator = new Random();

int random = randomGenerator.nextInt(nums.length);

Seqüència de números aleatoris

ArrayList<Integer> randomSequence = new ArrayList<Integer>();   
Random randomGenerator = new Random();

while (randomSequence.size() < nums.length) {
    int random = randomGenerator.nextInt(nums.length);
    if (!randomSequence.contains(random)) {
        randomSequence.add(random);
    }
}

Ternary Operator

n == 0 ? false : Math.log10(n) / Math.log10(3) % 1 == 0;