Data Structure

String

Char Array to String

char[] chars = {'a','b','c'};
String s = String.valueOf(chars);

StringBuilder

Insert at begining

sb.insert(0, "string_should_be_at_begining");

Append

sb.append("string_to_append");

Set to empty

sb.setLength(0);

List

Set

Delcare

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

Add

set.add(123);

Delete

set.remove(123);

Check contains

set.contains(123); // ture or false

Map

Declaration

Map<Integer, Integer> map = new HashMap<>();

Add

map.put(key, value);

Check contains

map.containsKey(key);
map.containsValue(value);

Update

map.put(key, map.getOrDefault(key, default_value) + 1);

Iteration for values

for (int v : map.values()) {
...
}

Interation for keys

for (int k : map.keySet()) {
    ....
}

Interation for element

map.forEach((k,v) -> {
  // Can't return anything here
});

for(Map.Entry<Integer, Interger> e : map.entrySet()) {
    int key = entry.getKey();
    int value = entry.getValue();
    // Can return here
}

Stack

inintial

Stack<Integer> s = new Stack();

Array of object

Stack<Integer>[] prev = new Stack[26];

// Make sure to initialize each stack in the array
for (int i = 0; i < 26; i++) {
    prev[i] = new Stack<>();
}