question: public static int prefixLength ( char [ ] s1 , char [ ] s2 ) { int len = 0 ; int max = Math . min ( s1 . length , s2 . length ) ; for ( int i = 0 ; i < max && s1 [ i ] == s2 [ i ] ; ++ i ) ++ len ; return len ; }
target:  Returns the length of the common prefix between s1 and s2. 