Algorithm dossier

  • Category: Graph
  • Worst-case complexity: O(n)
  • Approach: Iterative
  • Data structure: Graph
  • First formalised: 1950s

Why this snippet is Breadth-First Search

Why BFS. The head/tail pair on q is a FIFO queue: head++ dequeues the *oldest* node, so traversal expands outward in waves of equal hop-count from s. DFS would have used a stack (head and tail collapsed into one — push/pop from the same end). Use case. Shortest path in *unweighted* graphs falls out naturally from the wave-front order.

How to read a redacted algorithm

Algodle strips identifier names so the snippet has to be read for its shape: the control flow, the data structures it manipulates, the order in which it visits its input. Loops with two pointers crawling toward each other are usually search or partition. A recursion that splits its input in half and recurses on both halves is divide-and-conquer. A priority queue plus graph traversal is almost certainly Dijkstra, Prim, or A*. Six hint columns — category, complexity, approach, data structure, era — let you triangulate even when the snippet itself is opaque.