TikTok Interview Questions 2027: Walls and Gates & Asteroid Collision

For these two TikTok interview questions, name the pattern first: Walls and Gates (LeetCode 286) is multi-source BFS — queue all gates at once and expand outward; Asteroid Collision (LeetCode 735) is a stack simulation — resolve left-movers against the stack top. Both reward recognizing the pattern, then clean edge cases.

TikTok Interview Questions: What These Two Problems Test

This pair is commonly reported by candidates in TikTok coding rounds as a breadth check: two medium problems, two classic patterns. Walls and Gates tests whether you know the multi-source BFS trick — starting BFS from all gates simultaneously instead of from each room — which turns an O((mn)^2) brute force into O(mn). Asteroid Collision tests stack-based simulation with careful case analysis. Interviewers are testing pattern recognition plus clean handling of edge cases.

How to Solve These TikTok Interview Questions Step by Step

Walls and Gates (multi-source BFS):

• State the insight. "Instead of BFS from every empty room, I'll BFS once from all gates simultaneously — the first time we reach a room is via the shortest path."
• Initialize. Queue all gate cells with distance 0; treat walls as blocked.
• Expand. Pop a cell, visit its four neighbors: skip walls and visited rooms, set unvisited empty rooms to current distance + 1, and enqueue them.
• Result. Every reachable room now holds its distance to the nearest gate; unreachable rooms keep their INF marker.

Asteroid Collision (stack simulation):

• State the insight. "Only a left-moving asteroid meeting right-moving ones on the stack can collide — everything else just pushes."
• Iterate. For each asteroid: if it moves right (positive), push it. If it moves left (negative), resolve against the stack top while the top moves right.
• Resolve collisions. Compare absolute sizes: smaller explodes (pop and continue), equal sizes destroy each other (pop, and the incoming one is gone too), larger incoming destroys the top (pop and keep checking).
• Edge cases. A left-moving asteroid with an empty stack or a left-moving top simply pushes — no collision possible.

Example line: "Both problems are pattern problems — multi-source BFS collapses many searches into one, and the stack turns the collision cascade into a clean loop."

Common Mistakes

• Walls and Gates: BFS from each room. Correct but far too slow — the multi-source insight is the whole point.
• Asteroid Collision: forgetting the chain reaction. One incoming asteroid can destroy several stack tops; keep looping.
• Equal-size handling. Both explode — a frequently fumbled case.

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FAQ

What are the complexities? Both O(mn)/O(n) time respectively, with O(mn) and O(n) space.

Can Walls and Gates be solved with DFS? Not correctly for shortest distances — BFS's level order is what guarantees minimal distance.

How do I keep the collision cases straight? Write the three outcomes (smaller/equal/larger) as comments before coding the loop.

Which should I solve first in the interview? Asteroid Collision — it is more mechanical, which builds momentum for the BFS insight.

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