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