DSA · Theory

Breadth First Search or BFS for a Graph

Theory

237/810

Breadth First Search or BFS for a Graph

If you can teach Breadth First Search or BFS for a Graph using bus routes, you know it. If you only know the heading, you don’t.

Breadth First Search or BFS for a Graph order: meaning → BFS queue / DFS stack → one limit. Dumping ten features helps nobody.

Without Breadth First Search or BFS for a Graph, a city map gets messy and bus routes is hard to trust.

Zara ships Breadth First Search or BFS for a Graph in a city map. That is the use case worth saying.

If bus routes breaks under Breadth First Search or BFS for a Graph, check no visited set → infinite loop first.

After Breadth First Search or BFS for a Graph, Zara should still remember no visited set → infinite loop.

Close Breadth First Search or BFS for a Graph with: “If I skip it, bus routes goes wrong like this: no visited set → infinite loop.”

Exam tip

Say Breadth First Search or BFS for a Graph in one breath, then BFS queue / DFS stack, then no visited set → infinite loop.

Example

from collections import deque

graph = {1: [2, 3], 2: [4], 3: [], 4: []}

def bfs(start):
    seen, q = {start}, deque([start])
    order = []
    while q:
        u = q.popleft()
        order.append(u)
        for v in graph[u]:
            if v not in seen:
                seen.add(v)
                q.append(v)
    return order

print(bfs(1))

Breadth First Search or BFS for a Graph — bFS explores level by level using a queue.

Short notes

DefBreadth First Search or BFS for a Graph — Zara uses it for bus routes in a city map.
RuleBreadth First Search or BFS for a Graph → BFS queue / DFS stack.
RememberBreadth First Search or BFS for a Graph + Big-O (a city map).
UseBreadth First Search or BFS for a Graph in a city map (bus routes).
TrapBreadth First Search or BFS for a Graph — no visited set → infinite loop.
ExBreadth First Search or BFS for a Graph → bus routes.

from collections import deque graph = {1: [2, 3], 2: [4], 3: [], 4: []} def bfs(start): seen, q = {start}, deque([start]) order = [] while q: u = q.popleft() order.append(u) for v in graph[u]: if v not in seen: seen.add(v) q.append(v) return order print(bfs(1))

Breadth First Search or BFS for a Graph

Example

Short notes

Questions

Breadth First Search or BFS for a Graph

Example

Short notes

Questions