Mappings & Arrays

Mappings are hash tables: `mapping(address => uint256) public balances;`. They provide O(1) lookups but can't be iterated. Arrays can be fixed-size (`uint[5]`) or dynamic (`uint[]`). Dynamic arrays in storage can be pushed to and popped from. Important: mappings can only live in storage, not memory. A common pattern combines both: use a mapping for O(1) lookup and an array to track all keys for iteration.