Coding problem: Cheapest Delivery Route. A delivery network is given as an adjacency list: `graph` maps each depot name to a list of `[neighbor, cost]` pairs, one for each one-way road leaving that depot. Costs are non-negative integers. Return the lowest total cost of a route from `start` to `goal`, or `-1` if there's no route.

What the interviewer is looking for

• What a strong solution looks like: Dijkstra with a priority queue and lazy deletion (skip popped entries whose cost is stale). O((V + E) log V). For graphs this small an O(V^2) Dijkstra or Bellman-Ford is acceptable — ask about the trade-off.
• Think aloud. Interviewers are judging how you reason, not only the final code.
• Be ready for: Return the route as well as the cost (keep a parent map). What if some costs could be negative — why does Dijkstra break, and what would you use? Limit the route to at most k roads. Many queries against the same network — what would you precompute?

A structure to follow: Clarify, plan, code, test, analyse

1. Step 1

   Clarify

   Restate the problem, ask about empty input, duplicates and size limits, and agree on one example.

2. Step 2

   Plan

   Say the simple approach first and its cost, then the better one. Explain why before you type.

3. Step 3

   Code

   Write small named steps and talk through each. A clear solution beats a clever one.

4. Step 4

   Test

   Walk your own example through the code, then an edge case. Fix what you find calmly.

5. Step 5

   Analyse

   State time and space cost in plain words and say what you would change for much larger input.

This is guidance on shape, not a script. Use your own real experience and facts; never invent details to fill a gap. See the STAR guide and answer frameworks.

Common slips

• Starting to code before agreeing what the answer must be.
• Going silent when stuck instead of saying what you are trying.
• Never running your own example through the code.

How to practise it

Solve it once in the editor without hints, then explain your solution aloud in under two minutes.

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