Create count-the-number-of-good-subsequences.py

Author: kamyu104

Python/count-the-number-of-good-subsequences.py

@@ -0,0 +1,26 @@
+# Time:  O(26 * n)
+# Space: O(n)
+
+import collections
+
+
+# combinatorics
+class Solution(object):
+    def countGoodSubsequences(self, s):
+        """
+        :type s: str
+        :rtype: int
+        """
+        MOD = 10**9+7
+        fact, inv, inv_fact = [[1]*2 for _ in xrange(3)]
+        def nCr(n, k):
+            if not (0 <= k <= n):
+                return 0
+            while len(inv) <= n:  # lazy initialization
+                fact.append(fact[-1]*len(inv) % MOD)
+                inv.append(inv[MOD%len(inv)]*(MOD-MOD//len(inv)) % MOD)  # https://cp-algorithms.com/algebra/module-inverse.html
+                inv_fact.append(inv_fact[-1]*inv[-1] % MOD)
+            return (fact[n]*inv_fact[n-k] % MOD) * inv_fact[k] % MOD
+
+        cnt = collections.Counter(s)
+        return reduce(lambda total, k: (total+reduce(lambda total, x: total*(1+nCr(x, k))%MOD, cnt.itervalues(), 1)-1)%MOD, xrange(1, max(cnt.itervalues())+1), 0)