Leetcode 1129. Shortest Path with Alternating Colors

Find the shortest distance from node 0 to every node in a directed graph whose edges are colored red or blue, with the constraint that consecutive edges along the path must alternate colors; return -1 for nodes that cannot be reached under this constraint. This is typically solved by BFS over states that pair a node with the color of the incoming edge to enforce alternation and avoid revisiting the same state.