Create arithmetic-slices.py

Author: kamyu104

Diff for: Python/arithmetic-slices.py

@@ -0,0 +1,44 @@
+# Time:  O(n)
+# Space: O(1)
+
+# A sequence of number is called arithmetic if it consists of at least three elements
+# and if the difference between any two consecutive elements is the same.
+#
+# For example, these are arithmetic sequence:
+#
+# 1, 3, 5, 7, 9
+# 7, 7, 7, 7
+# 3, -1, -5, -9
+# The following sequence is not arithmetic.
+#
+# 1, 1, 2, 5, 7
+#
+# A zero-indexed array A consisting of N numbers is given. A slice of that array is any pair
+# of integers (P, Q) such that 0 <= P < Q < N.
+#
+# A slice (P, Q) of array A is called arithmetic if the sequence:
+# A[P], A[p + 1], ..., A[Q - 1], A[Q] is arithmetic. In particular, this means that P + 1 < Q.
+#
+# The function should return the number of arithmetic slices in the array A.
+#
+# Example:
+#
+# A = [1, 2, 3, 4]
+#
+# return: 3, for 3 arithmetic slices in A: [1, 2, 3], [2, 3, 4] and [1, 2, 3, 4] itself.
+
+class Solution(object):
+    def numberOfArithmeticSlices(self, A):
+        """
+        :type A: List[int]
+        :rtype: int
+        """
+        res, i = 0, 0
+        while i+2 < len(A):
+            start = i
+            while i+2 < len(A) and A[i+2] + A[i] == 2*A[i+1]:
+                res += i - start + 1
+                i += 1
+            i += 1
+  
+        return res