LeetCode 207: Course Schedule — Step-by-Step Visual Trace

Medium — Graph | Topological Sort | BFS | Cycle Detection The Problem Determine if it's possible to finish all courses given a list of prerequisite pairs, where each pair [a,b] indicates course a requires course b to be completed first. Approach Use topological sorting with Kahn's algorithm to detect cycles in the course dependency graph. Build a graph and track in-degrees, then process courses with no prerequisites first, removing edges and checking if all courses can eventually be processed. Time: O(V + E) · Space: O(V + E) Code class Solution : def canFinish ( self , numCourses : int , prerequisites : List [ List [ int ]]) -> bool : graph = { i : [] for i in range ( numCourses )} in_degree = [ 0 ] * numCourses # Construct the graph and count in-degrees for course , prereq in prerequisites : graph [ prereq ]. append ( course ) in_degree [ course ] += 1 # Initialize a queue with nodes having in-degree zero queue = collections . deque ( [ course for course , degree in enumerate ( in_de