Create maximum-difference-between-even-and-odd-frequency-ii.py

Author: kamyu104

Python/maximum-difference-between-even-and-odd-frequency-ii.py

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+# Time:  O(d^2 * n)
+# Space: O(n)
+
+# prefix sum, two pointers, sliding window
+class Solution(object):
+    def maxDifference(self, s, k):
+        """
+        :type s: str
+        :type k: int
+        :rtype: int
+        """
+        def diff(x, y):
+            prefix1, prefix2, prefix = [0]*(len(s)+1), [0]*(len(s)+1), [0]*(len(s)+1)
+            for i in xrange(len(s)):
+                prefix1[i+1] = prefix1[i]+int(s[i] == x)
+                prefix2[i+1] = prefix2[i]+int(s[i] == y)
+                prefix[i+1] = prefix[i]+(int(s[i] == x)-int(s[i] == y))
+            result = float("-inf")
+            mn = [[float("inf")]*2 for _ in xrange(2)]
+            left = 0
+            for right in xrange(k-1, len(s)):
+                while k <= right-left+1 and prefix1[right+1]-prefix1[left] and prefix2[right+1]-prefix2[left]:
+                    x, y = prefix1[left]%2, prefix2[left]%2
+                    mn[x][y] = min(mn[x][y], prefix[left])
+                    left += 1
+                x, y = prefix1[right+1]%2, prefix2[right+1]%2
+                result = max(result, prefix[right+1]-mn[x^1][y])
+            return result
+        
+        lookup = set(s)
+        return max(diff(x, y) for x in lookup for y in lookup if x != y)