61. K-Nearest Neighbors: Judge by Your Company

Every other algorithm we've covered so far actually learns something during training. It builds a model, adjusts weights, grows a tree. KNN does none of that. It stores the entire training dataset. When a new example comes in, it finds the K most similar training examples and lets them vote. Majority wins. That's it. No training. No model. Just memory and distance. And yet it works surprisingly well on many problems. Understanding why it works, and more importantly when it fails, will make you a