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Breadth-first Search

Breadth-first Search (BFS) is one of the well-known brute-force search algorithms which has been widely used. The basic idea is just like its name: do the searching by expansion. Staring with the sources, the expansion diffuses layer by layer. In contrast, another common search algorithm is Depth-first Search (DFS) which focuses on one route first until reaching the end, then starts the next possible route.

So one way to understand BFS is that: BFS searches all the possible routes at the same time. And for each route, BFS usually makes the same amount of progress. In the middle of the search, once a route becomes invalid, it can be dropped right away.

So for BFS,

1. the minimum distance can be obtained since the shortest routes can be always found before other routes;

2. to avoid redundant search, need to make sure each position (state) is only searched once. [DFS also needs this].

3. a queue is usually used for the search, to easier realize the layer-by-layer expansion.


Example

Easy Level

Medium Level

Hard Level

Interview Questions


Upper Layer

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