feat: add swift implementation to lcof problem: No.45 (doocs#2926)

Author: klever34

lcof/面试题45. 把数组排成最小的数/README.md

@@ -171,6 +171,18 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func minNumber(_ nums: [Int]) -> String {
+        let sortedNums = nums.map { String($0) }
+                             .sorted { $0 + $1 < $1 + $0 }
+        return sortedNums.joined()
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题45. 把数组排成最小的数/Solution.swift

@@ -0,0 +1,7 @@
+class Solution {
+    func minNumber(_ nums: [Int]) -> String {
+        let sortedNums = nums.map { String($0) }
+                             .sorted { $0 + $1 < $1 + $0 }
+        return sortedNums.joined()
+    }
+}