swift: add «724. Find Pivot Index»

Signed-off-by: Matej Focko <me@mfocko.xyz>

swift/find-pivot-index.swift

@@ -0,0 +1,18 @@
+class Solution {
+    func pivotIndex(_ nums: [Int]) -> Int {
+        var fromLeft = 0
+        var fromRight = nums.reduce(0, +)
+
+        for (i, x) in nums.enumerated() {
+            fromRight -= x
+
+            if fromLeft == fromRight {
+                return i
+            }
+
+            fromLeft += x
+        }
+
+        return -1
+    }
+}