Song Sequence: Reconstruct Playlist Order from Partial Overlap Clues

Problem You are given a list of song pairs (a, b) meaning song a was played immediately before song b in a playlist. Reconstruct the full playlist order. Assume the input forms a single valid sequence with no ambiguity. Follow-ups How do you identify the starting song (the one that never appears as a "next" song)? What if the pairs form a cycle — how would you detect and report that? If some pairs are missing (gaps in the sequence), how do you handle discontinuous playlists? Extend to reconstruc…