Create arithmetic_slices.py

Author: zirea3l

dynamic_programming/arithmetic_slices.py

@@ -0,0 +1,74 @@
+"""
+    An integer array is called arithmetic if it 
+    consists of at least three elements and if 
+    the difference between any two consecutive 
+    elements is the same.
+
+    Given an integer array nums, 
+    return the number of 
+    arithmetic subarrays of nums.
+
+    A subarray is a contiguous 
+    subsequence of the array.
+
+"""
+
+class ArithmeticSlices:
+    def numberofarithmeticslices(self, nums):
+        
+        """
+        This defines a class and a function.  
+        `nums` is input list of integers.
+        """
+        n = len(nums)
+        
+        """
+        We store the length of the 
+        array nums in variable n
+        """
+        if n < 3:
+            return 0
+        
+        total = 0
+        curr = 0
+        
+        """
+        An *arithmetic slice* must have **at least 3 numbers**.
+
+        So, if the array has fewer than 3 elements,  
+        no arithmetic slices are possible — immediately return `0`.
+        """
+        
+        for i in range(2, n):
+            if nums[i] - nums[i - 1] == nums[i - 1] - nums[i - 2]:
+                curr += 1
+                total += curr
+            else:
+                curr = 0
+                
+        """
+        We start iterating from index `2` 
+        because we need **three elements** 
+        (`nums[i-2], nums[i-1], nums[i]`) 
+        to check if they form an arithmetic pattern.
+
+        So at each step, 
+        we’re looking at a triplet ending at index `i`.
+        """
+        
+        return total
+
+"""
+test_cases = [
+        #  Basic cases
+        ([1, 2, 3, 4], 3),      # [1,2,3], [2,3,4], [1,2,3,4]
+        ([1, 3, 5, 7, 9], 6),   # all diffs = 2; 
+                                total slices = 6
+        
+        #  Edge cases
+        ([1, 2], 0),            # less than 3 elements → 0
+        ([1, 1, 1], 1),         # [1,1,1] itself is arithmetic
+        ([1], 0),               # single element
+        ([], 0),                # empty array
+        ]
+"""