feat: add swift implementation to lcof problem: No.38 (doocs#2919)

Author: klever34

lcof/面试题38. 字符串的排列/README.md

@@ -283,6 +283,43 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    private var ans: [String] = []
+    private var cs: [Character] = []
+
+    func permutation(_ s: String) -> [String] {
+        cs = Array(s)
+        dfs(0)
+        return ans
+    }
+
+    private func dfs(_ i: Int) {
+        if i == cs.count - 1 {
+            ans.append(String(cs))
+            return
+        }
+        var vis: Set<Character> = []
+        for j in i..<cs.count {
+            if !vis.contains(cs[j]) {
+                vis.insert(cs[j])
+                swap(i, j)
+                dfs(i + 1)
+                swap(i, j)
+            }
+        }
+    }
+
+    private func swap(_ i: Int, _ j: Int) {
+        let t = cs[i]
+        cs[i] = cs[j]
+        cs[j] = t
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题38. 字符串的排列/Solution.swift

@@ -0,0 +1,32 @@
+class Solution {
+    private var ans: [String] = []
+    private var cs: [Character] = []
+
+    func permutation(_ s: String) -> [String] {
+        cs = Array(s)
+        dfs(0)
+        return ans
+    }
+
+    private func dfs(_ i: Int) {
+        if i == cs.count - 1 {
+            ans.append(String(cs))
+            return
+        }
+        var vis: Set<Character> = []
+        for j in i..<cs.count {
+            if !vis.contains(cs[j]) {
+                vis.insert(cs[j])
+                swap(i, j)
+                dfs(i + 1)
+                swap(i, j)
+            }
+        }
+    }
+
+    private func swap(_ i: Int, _ j: Int) {
+        let t = cs[i]
+        cs[i] = cs[j]
+        cs[j] = t
+    }
+}