TCP segment reassembly

Reassemble a byte stream from out-of-order TCP-style segments. You are given a list of [offset, data] segments where offset is the 0-based byte position and data is a string written starting at that offset. Segments may arrive out of order and may overlap; for any overlapping byte position the FIRST segment to cover it wins (later writes to an already-filled byte are ignored). Return the longest contiguous string starting at offset 0. If offset 0 is never written, return an empty string.