text: If current prefix satisfies the given conditions
code: if len ( k ) == 1 or ( k [ 0 ] == k [ - 2 ] and k [ - 1 ] - k [ - 2 ] == 1 ) or ( k [ 0 ] == 1 and k [ 1 ] == k [ - 1 ] ) : NEW_LINE INDENT ans = index NEW_LINE DEDENT