Count Binary Substrings

Group consecutive equal bits into runs. For each adjacent pair of runs with lengths a and b, exactly min(a, b) valid substrings use that boundary: choose one through min(a,b) characters from each side, yielding equal counts of zeroes and ones in contiguous groups. Scan once with previousRun and currentRun. When the bit changes, add min(previousRun, currentRun), assign previousRun = currentRun, and reset currentRun to one; after the scan add the last pair's contribution. This is simpler than enumerating substrings and makes the proof local to each boundary. Check a single-character string, all equal bits, and alternating bits. Time is O(n), space is O(1).