Minimum window substring: s="ADOBECODEBANC" t="ABC"
===================================================

right=5 'C'  window="ADOBEC" is VALID
    new best "ADOBEC" (length 6)
    drop 'A' -> window="DOBEC" no longer valid, missing=1
right=10 'A'  window="DOBECODEBA" is VALID
    drop 'D' -> window="OBECODEBA" still valid
    drop 'O' -> window="BECODEBA" still valid
    drop 'B' -> window="ECODEBA" still valid
    drop 'E' -> window="CODEBA" still valid
    drop 'C' -> window="ODEBA" no longer valid, missing=1
right=12 'C'  window="ODEBANC" is VALID
    drop 'O' -> window="DEBANC" still valid
    drop 'D' -> window="EBANC" still valid
    new best "EBANC" (length 5)
    drop 'E' -> window="BANC" still valid
    new best "BANC" (length 4)
    drop 'B' -> window="ANC" no longer valid, missing=1

answer = "BANC"