Add function to count arithmetic slices

Author: jyoung2419

leetcode3/정진영/413. Arithmetic Slices

@@ -0,0 +1,21 @@
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var numberOfArithmeticSlices = function(nums) {
+    let count = 0;
+    let ans = 0;
+
+    if (nums.length <= 2) return 0;
+
+    for (let i = 2; i<nums.length; i++){
+        if ((nums[i] - nums[i-1]) == (nums[i-1] - nums[i-2])){
+            count++;
+            ans += count;
+        } else {
+            count = 0;
+        }
+    }
+
+    return ans;
+};