diff --git a/Python/numbers-at-most-n-given-digit-set.py b/Python/numbers-at-most-n-given-digit-set.py
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+# Time:  O(logn)
+# Space: O(logn)
+
+# We have a sorted set of digits D, a non-empty subset of
+# {'1','2','3','4','5','6','7','8','9'}.  (Note that '0' is not included.)
+#
+# Now, we write numbers using these digits, using each digit as many times as we want.
+# For example, if D = {'1','3','5'}, we may write numbers such as '13', '551', '1351315'.
+#
+# Return the number of positive integers that can be written (using the digits of D) 
+# that are less than or equal to N.
+#
+# Example 1:
+#
+# Input: D = ["1","3","5","7"], N = 100
+# Output: 20
+# Explanation: 
+# The 20 numbers that can be written are:
+# 1, 3, 5, 7, 11, 13, 15, 17, 31, 33, 35, 37, 51, 53, 55, 57, 71, 73, 75, 77.
+# Example 2:
+#
+# Input: D = ["1","4","9"], N = 1000000000
+# Output: 29523
+# Explanation: 
+# We can write 3 one digit numbers, 9 two digit numbers, 27 three digit numbers,
+# 81 four digit numbers, 243 five digit numbers, 729 six digit numbers,
+# 2187 seven digit numbers, 6561 eight digit numbers, and 19683 nine digit numbers.
+# In total, this is 29523 integers that can be written using the digits of D.
+#
+# Note:
+# - D is a subset of digits '1'-'9' in sorted order.
+# - 1 <= N <= 10^9
+
+class Solution(object):
+    def atMostNGivenDigitSet(self, D, N):
+        """
+        :type D: List[str]
+        :type N: int
+        :rtype: int
+        """
+        str_N = str(N)
+        set_D = set(D)
+        result = sum(len(D)**i for i in xrange(1, len(str_N)))
+        i = 0
+        while i < len(str_N):
+            result += sum(c < str_N[i] for c in D) * (len(D)**(len(str_N)-i-1))
+            if str_N[i] not in set_D:
+                break
+            i += 1
+        return result + int(i == len(str_N))