Create minimum-number-of-operations-to-make-string-sorted.py

Author: kamyu104

Python/minimum-number-of-operations-to-make-string-sorted.py

@@ -0,0 +1,28 @@
+# Time:  O(26 * n) = O(n)
+# Space: O(26) = O(1)
+
+inv = [0, 1]
+
+
+class Solution(object):
+    def makeStringSorted(self, s):  # count of prev_permutation
+        """
+        :type s: str
+        :rtype: int
+        """
+        
+        def inverse(n, m):
+            i = len(inv)
+            while len(inv) <= n:  # lazy initialization
+                inv.append(inv[m%i]*(m-m//i) % m)  # https://cp-algorithms.com/algebra/module-inverse.html
+                i += 1
+            return inv[n]
+    
+        MOD = 10**9+7
+        count, result, comb_total = [0]*26, 0, 1
+        for i in reversed(xrange(len(s))):
+            num = ord(s[i])-ord('a') 
+            count[num] += 1
+            comb_total = (comb_total*(len(s)-i))*inverse(count[num], MOD)
+            result = (result + (comb_total*sum(count[:num]))*inverse(len(s)-i, MOD)) % MOD
+        return result