feat: add swift implementation to lcci problem: No.16.21 (#2764)

Author: klever34

lcci/16.21.Sum Swap/README.md

@@ -148,6 +148,37 @@ function findSwapValues(array1: number[], array2: number[]): number[] {
 }
 ```
 
+```swift
+class Solution {
+    func findSwapValues(_ array1: [Int], _ array2: [Int]) -> [Int] {
+        var s1 = 0, s2 = 0
+        var set = Set<Int>()
+
+        for x in array1 {
+            s1 += x
+        }
+        for x in array2 {
+            s2 += x
+            set.insert(x)
+        }
+
+        let diff = s1 - s2
+        if diff % 2 != 0 {
+            return []
+        }
+        let target = diff / 2
+
+        for a in array1 {
+            let b = a - target
+            if set.contains(b) {
+                return [a, b]
+            }
+        }
+        return []
+    }
+}
+```
+
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 <!-- end -->

lcci/16.21.Sum Swap/README_EN.md

@@ -154,6 +154,37 @@ function findSwapValues(array1: number[], array2: number[]): number[] {
 }
 ```
 
+```swift
+class Solution {
+    func findSwapValues(_ array1: [Int], _ array2: [Int]) -> [Int] {
+        var s1 = 0, s2 = 0
+        var set = Set<Int>()
+
+        for x in array1 {
+            s1 += x
+        }
+        for x in array2 {
+            s2 += x
+            set.insert(x)
+        }
+
+        let diff = s1 - s2
+        if diff % 2 != 0 {
+            return []
+        }
+        let target = diff / 2
+
+        for a in array1 {
+            let b = a - target
+            if set.contains(b) {
+                return [a, b]
+            }
+        }
+        return []
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- end -->

lcci/16.21.Sum Swap/Solution.swift

@@ -0,0 +1,28 @@
+class Solution {
+    func findSwapValues(_ array1: [Int], _ array2: [Int]) -> [Int] {
+        var s1 = 0, s2 = 0
+        var set = Set<Int>()
+        
+        for x in array1 {
+            s1 += x
+        }
+        for x in array2 {
+            s2 += x
+            set.insert(x)
+        }
+        
+        let diff = s1 - s2
+        if diff % 2 != 0 {
+            return []
+        }
+        let target = diff / 2
+        
+        for a in array1 {
+            let b = a - target
+            if set.contains(b) {
+                return [a, b]
+            }
+        }
+        return []
+    }
+}