java

int[] nums

1. nums.length
2. nums[0]
3. Arrays.sort(nums)

#Integer num

1. Num1.equals(num2)

#String str

1. str.length();
2. str.charAt(i);
3. str.equalsIgnoreCase(str3)
4. str.substring(i,j) -> both included
5. String.valueOf(s)
6. str.toLowerCase()
7. String[] arrOfStr = str.split("@")
8. char[] arrOfChar = str.toCharArray()
9. str.replaceAll("\s", "");

#List<> list

1. list.get(i);
2. list.size();
3. list.add(p);
4. list.remove(index) # remove at index #O(n)
5. list.add(index, element); # add value at index

#Queue

1. Queue queue = new ArrayDeque();
2. queue.add(i) #void
3. queue.poll() #return
4. queue.isEmpty() #check empty

#Stack

1. Stack stack = new Stack();
2. stack.push(i) #void
3. stack.pop() #return
4. stack.peek() #peek top element
5. stack.empty() #check empty

#Map (Time complexity is O(1) for all)

1. Map<Integer, Integer> map = new HashMap<>();
2. map.put(key, value); #void
3. map.get(key) #Integer
4. map.values() #Collection #to get all value
5. map.keySet() #Set
6. map.remove(key) # remove key
7. for(Map.Entry<String,String> entry : map.entrySet()){ entry.getKey() , entry.getValue()}

#PriorityQueue

1. PriorityQueue pq = new PriorityQueue();
2. pq.peek() #peek top element
3. pq.poll() # return head and remove head
4. Iterator value = queue.iterator(); while (value.hasNext()) { System.out.println(value.next()); }