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Big-O & Data Structures

Complexity classes and structure trade-offs.

Complexity classes (best → worst)

```
O(1)
```
constant — array index
```
O(log n)
```
logarithmic — binary search
```
O(n)
```
linear — scan once
```
O(n log n)
```
good sorting
```
O(n²)
```
nested loops — avoid at scale

Structure operations (average)

ArrayListget O(1), add-end O(1), insert O(n)
LinkedListadd-ends O(1), get O(n)
HashMap / HashSetget/put/contains O(1)
TreeMap / TreeSetget/put O(log n), sorted
ArrayDeque (stack/queue)push/pop/poll O(1)

Sorting & searching

```
Arrays.sort(a) / Collections.sort(l)
```
O(n log n)
```
Arrays.binarySearch(sorted, key)
```
O(log n)

list.sort(Comparator.comparingInt(X::n))