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hardDSA

When would you use BFS over Dijkstra for shortest paths?

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01

Understand the problem

Compare BFS and Dijkstra.

graphsshortest-path
02

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Sketch your approach before reading the solution — that's what interviews test.

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03

Study the solution

Use plain BFS when edges are unweighted (or all equal weight) — it finds shortest paths in O(V+E). Use Dijkstra (with a heap) for non-negative weighted graphs. For negative weights use Bellman-Ford.

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04

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BFS shortest path (unweighted)
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from collections import deque

def bfs_shortest(graph, src, dst):
    q = deque([(src, 0)])
    seen = {src}
    while q:
        node, dist = q.popleft()
        if node == dst:
            return dist
        for nb in graph[node]:
            if nb not in seen:
                seen.add(nb)
                q.append((nb, dist + 1))
    return -1


# --- demo ---  unweighted graph
graph = {0: [1, 2], 1: [0, 3], 2: [0, 3], 3: [1, 2, 4], 4: [3]}
print(bfs_shortest(graph, 0, 4))   # 3
05

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