feat: add swift solution to lc problem: No.0137 (#892)

No.0137.Single Number II

Author: Qiu-IT

Diff for: solution/0100-0199/0137.Single Number II/README.md

@@ -122,6 +122,26 @@ public:
 };
 ```
 
+### **Swift**
+
+```swift
+class Solution {
+  func singleNumber(_ nums: [Int]) -> Int {
+    var a = nums.sorted()
+    var n = a.count
+    for i in stride(from: 0, through: n - 1, by: 3) {
+      if i == n - 1 {
+        return a[i]
+      }
+      else if a[i] != a[i + 1] {
+        return a[i]
+      }
+    }
+    return 0
+  }
+}
+```
+
 ### **...**
 
 ```

Diff for: solution/0100-0199/0137.Single Number II/README_EN.md

@@ -101,6 +101,26 @@ public:
 };
 ```
 
+### **Swift**
+
+```swift
+class Solution {
+  func singleNumber(_ nums: [Int]) -> Int {
+    var a = nums.sorted()
+    var n = a.count
+    for i in stride(from: 0, through: n - 1, by: 3) {
+      if i == n - 1 {
+        return a[i]
+      }
+      else if a[i] != a[i + 1] {
+        return a[i]
+      }
+    }
+    return 0
+  }
+}
+```
+
 ### **...**
 
 ```

Diff for: solution/0100-0199/0137.Single Number II/Solution.swift

@@ -0,0 +1,15 @@
+class Solution {
+  func singleNumber(_ nums: [Int]) -> Int {
+    var a = nums.sorted()
+    var n = a.count
+    for i in stride(from: 0, through: n - 1, by: 3) {
+      if i == n - 1 {
+        return a[i]
+      }
+      else if a[i] != a[i + 1] {
+        return a[i]
+      }
+    }
+    return 0
+  }
+}