Python3 program to find n - th number containing only 4 and 7.
def reverse ( s ) : NEW_LINE INDENT if len ( s ) == 0 : NEW_LINE INDENT return s NEW_LINE DEDENT else : NEW_LINE INDENT return reverse ( s [ 1 : ] ) + s [ 0 ] NEW_LINE DEDENT DEDENT def findNthNo ( n ) : NEW_LINE INDENT res = " " ; NEW_LINE while ( n >= 1 ) : NEW_LINE DEDENT