292. Number of Sub-arrays With Odd Sum

Author: ahmlvs

1524. Number of Sub-arrays With Odd Sum/solution.py

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+# Given an array of integers arr, return the number of subarrays with an odd sum.
+
+# Since the answer can be very large, return it modulo 109 + 7.
+
+ 
+
+# Example 1:
+
+# Input: arr = [1,3,5]
+# Output: 4
+# Explanation: All subarrays are [[1],[1,3],[1,3,5],[3],[3,5],[5]]
+# All sub-arrays sum are [1,4,9,3,8,5].
+# Odd sums are [1,9,3,5] so the answer is 4.
+# Example 2:
+
+# Input: arr = [2,4,6]
+# Output: 0
+# Explanation: All subarrays are [[2],[2,4],[2,4,6],[4],[4,6],[6]]
+# All sub-arrays sum are [2,6,12,4,10,6].
+# All sub-arrays have even sum and the answer is 0.
+# Example 3:
+
+# Input: arr = [1,2,3,4,5,6,7]
+# Output: 16
+ 
+
+# Constraints:
+
+# 1 <= arr.length <= 105
+# 1 <= arr[i] <= 100
+
+from typing import List
+
+class Solution:
+    def numOfSubarrays(self, arr: List[int]) -> int:
+        n = len(arr)
+        odd_count = 0
+        prefix_sum = 0
+        mod = 10**9+7
+
+        for el in arr:
+            prefix_sum += el
+            odd_count += prefix_sum % 2
+        
+        odd_count += (n - odd_count) * odd_count
+
+        return odd_count % mod

1524. Number of Sub-arrays With Odd Sum/test_solution.py

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+import unittest
+from solution import Solution
+
+
+class TestSolution(unittest.TestCase):
+
+    def test_case_1(self):
+        sol = Solution()
+        self.assertEqual(sol.numOfSubarrays([1,3,5]), 4)
+    
+    def test_case_2(self):
+        sol = Solution()
+        self.assertEqual(sol.numOfSubarrays([2,4,6]), 0)
+    
+    def test_case_3(self):
+        sol = Solution()
+        self.assertEqual(sol.numOfSubarrays([1,2,3,4,5,6,7]), 16)