Graphs & Grids

If you only have time to drill one coding pattern before a DoorDash loop, drill grid traversal. It is the most-cited family in candidate reports, and the single problem that shows up most is a multi-source BFS in a logistics framing. You are handed a city grid, some cells hold an open DashMart, some are blocked roads, and you have to label every other block with its distance to the nearest DashMart. Underneath that framing it is a Premium question, or 01 Matrix, or Shortest Distance from All Buildings. Same algorithm, three names, one round you cannot afford to fumble.

The reason it recurs is that it maps cleanly onto what DoorDash builds. A dispatcher asking which Dasher is closest to a pickup, an ops tool shading a map by delivery reach, a batching engine grouping orders by travel time: all of it is shortest-distance-on-a-grid underneath. So the interviewer can pull the prompt straight from the product and it still tests the algorithm they want. Your job is to hear "nearest DashMart through blocked roads" and immediately think multi-source BFS, then write it without freezing when the first run throws an index error.

The DoorDash angle

Every grid problem here arrives in marketplace terms, and that framing is the first thing you have to see through. DoorDash rarely says "matrix" or "BFS." It says DashMarts, Dashers, blocked roads, delivery zones. The translation step is part of the grade. Name the grid (the map), the cells (city blocks), the sources (every DashMart at once), and the obstacles (cells you cannot enter), and the algorithm falls out. Candidates who treat the story as noise and candidates who can restate it as a graph in one sentence are easy to tell apart, and only one of them gets to the follow-up.

DoorDashPremium lesson · Coding round

The 3 DoorDash problems are below.

1. 1A Premium questionCoding · Medium · Signature problem
2. 2A Premium questionCoding · Hard · Frequently asked
3. 3A Premium questionCoding · Medium

Each with hints, the follow-up and a tested solution.

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From 166 DoorDash reports, forum threads behind logins and write-ups in Chinese.

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