Did yesterday's daily

Author: Zanger67

my-submissions/m1524 v0.py

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+class Solution:
+    def numOfSubarrays(self, arr: List[int]) -> int:
+        for i in range(1, len(arr)) :
+            arr[i] += arr[i - 1]
+
+        arr[0] = (arr[0] % 2, (arr[0] + 1) % 2)     # (# odds, # evens)
+        for i in range(1, len(arr)) :
+            arr[i] = (arr[i - 1][0] + arr[i] % 2, arr[i - 1][1] + (arr[i] + 1) % 2)
+
+        return arr[-1][0] + arr[-1][1] * arr[-1][0] % (10 ** 9 + 7)

my-submissions/m1524 v1.py

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+class Solution:
+    def numOfSubarrays(self, arr: List[int]) -> int:
+        for i in range(1, len(arr)) :
+            arr[i] += arr[i - 1]
+
+        output = 0
+        evens, odds = 1, 0
+        for i in range(len(arr)) :
+            if arr[i] % 2 : # odd
+                output += evens
+                odds += 1
+            else :
+                output += odds
+                evens += 1
+        return output % (10 ** 9 + 7)

my-submissions/m1524 v2.py

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+class Solution:
+    def numOfSubarrays(self, arr: List[int]) -> int:
+        output = 0
+        pref_sum = 0
+        evens, odds = 1, 0
+        for num in arr :
+            pref_sum += num
+            if pref_sum % 2 : # odd
+                output += evens
+                odds += 1
+            else :
+                output += odds
+                evens += 1
+        return output % (10 ** 9 + 7)

my-submissions/m1524.md

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+## V0
+
+Initial attempt where I make each iteration by pass and reuse the existing array. Was inefficient I realized but the math worked. The process consisted of...
+
+1. Turning arr into a prefix sum array
+2. Replacing each index of arr with a tuple containing the prefix count of the number of items prior that had an odd or even prefix sum
+3. Returning the number of odd prefix sums plus the number of odds times the number of evens (since those can be matched up to get a subarray).
+
+## V1
+
+Much improved version that insteads sums the cases during the odds and evens iteration rather than replacing the array values
+
+## V2
+
+Improved version off of V1 that removes the need for the first prefix sum pass as it does all calculations within a single pass.