feat: add swift implementation to lcof2 problem: No.012 (doocs#2982)

Author: klever34

lcof2/剑指 Offer II 012. 左右两边子数组的和相等/README.md

@@ -226,6 +226,26 @@ int pivotIndex(int* nums, int numsSize) {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func pivotIndex(_ nums: [Int]) -> Int {
+        var leftSum = 0
+        var rightSum = nums.reduce(0, +)
+        
+        for i in 0..<nums.count {
+            rightSum -= nums[i]
+            if leftSum == rightSum {
+                return i
+            }
+            leftSum += nums[i]
+        }
+        return -1
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 012. 左右两边子数组的和相等/Solution.swift

@@ -0,0 +1,15 @@
+class Solution {
+    func pivotIndex(_ nums: [Int]) -> Int {
+        var leftSum = 0
+        var rightSum = nums.reduce(0, +)
+        
+        for i in 0..<nums.count {
+            rightSum -= nums[i]
+            if leftSum == rightSum {
+                return i
+            }
+            leftSum += nums[i]
+        }
+        return -1
+    }
+}