- MongoDB – Text Search
Text indexes and field weights, the one-per-collection limit, and the point where you should stop and reach for a real search engine.
- LeetCode 203 – Remove Linked List Elements
The problem that proves the dummy-head rule problem 83 stated. Here the head CAN be removed, repeatedly, so the dummy stops being a preference and becomes what makes the code short. And you must return dummy.next, never head.
- LeetCode 202 – Happy Number
Linked List Cycle with the list replaced by a function. No nodes, no next pointer, and Floyd's algorithm works anyway — which is the point: cycle detection needs a successor function, not a data structure. Plus why the sequence must terminate at all.
- LeetCode 200 – Number of Islands
The most common graph question in interviews, and it does not look like one — recognising that a grid is a graph is most of what is being tested. Count starts and erase the island so it cannot be counted twice. Why you must mark visited before recursing, the input-mutation trade to say out loud, and when the recursion depth forces BFS.
- LeetCode 199 – Binary Tree Right Side View
The clearest illustration that how you traverse and what you record are independent: level order with one line changed. The visible node may be a LEFT child — rightmost at its depth, not on the right spine — which is what any walk-down-the-right-side answer gets wrong.