I had 800-something LeetCode problems solved when I interviewed at Amazon. I figured I was ready. Two-pointer, sliding window, DP, graphs — I'd drilled all of it. I could smell a "this is a heap problem" from the second line of a prompt. Then they handed me a repository. Not a blank editor — an actual codebase that was partly built, and the task was to implement a couple of new features and debug the existing ones that were broken. Real code I hadn't written, a test suite I had to get green, ...