Update LongestSubstringNonrepeat.java

Submission Result: Compile Error

Line 18: cannot find symbol: variable S

Submission Result: Runtime Error

Last executed input:	L 18 and L 19 "wlrbbmqbhcdarzowkkyhiddqscdxrjmowfrxsjybldbefsarcbynecdyggxxpklorellnmpapqfwkhopkmco"