Length of Longest Non-Repeating Substring
Jelly, 6 bytes
ẆQƑƇṪL

A monadic link accepting a list of characters (or whatever), which yields the length.
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How?
ẆQƑƇṪL - Link: list
Ẇ      - all sublists (ordered by length)
   Ƈ   - filter keep those which are:
  Ƒ    -   invariant under:
 Q     -     deduplication
    Ṫ  - tail
     L - length