problems(swift): add “1287. Element Appearing More Than 25% In Sorted Array”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/swift/element-appearing-more-than-25-in-sorted-array.swift

@@ -0,0 +1,22 @@
+class Solution {
+    func findSpecialInteger(_ arr: [Int]) -> Int {
+        let threshold = arr.count / 4
+
+        var last = -1
+        var counter = 0
+        for x in arr {
+            if last != x {
+                counter = 1
+                last = x
+            } else {
+                counter += 1
+            }
+
+            if counter > threshold {
+                return last
+            }
+        }
+
+        return -1
+    }
+}