Implement a Hash Map from Scratch

To implement a HashMap from scratch, I would start by defining a Node class for our chaining mechanism, containing key, value, and a next pointer. The core structure will be an array of these nodes. First, I need a hash function. A good approach is to take the key's integer representation, multiply it by a prime number to mix bits, and then apply the modulo operator with the current array size. This ensures indices stay within bounds while distributing keys evenly. Next, for the put operation, I calculate the index using the hash function. If that bucket is empty, I insert the new node. If not, I traverse the linked list at that index. If the key already exists, I update the value; otherwise, I append the new node to the end of the chain. For get, I follow the same path: hash the key, find the bucket, and traverse until I match the key or reach the end. Remove works similarly by unlinking the node. A critical aspect is handling collisions effectively. Since we use chaining, multiple keys map to the same index but reside in separate list nodes. Finally, I must consider resizing. If the number of elements divided by the array size exceeds a load factor, say 0.75, I double the array size and rehash all existing entries to maintain O(1) performance. This approach mirrors how Airbnb handles scalable user session storage, ensuring low latency even under heavy load.