feat: add swift implementation to lcof problem: No.31 (doocs#2901)

Author: klever34

lcof/面试题31. 栈的压入、弹出序列/README.md

@@ -208,6 +208,27 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func validateStackSequences(_ pushed: [Int], _ popped: [Int]) -> Bool {
+        var stack = [Int]()
+        var j = 0
+        
+        for v in pushed {
+            stack.append(v)
+            while !stack.isEmpty && stack.last == popped[j] {
+                stack.removeLast()
+                j += 1
+            }
+        }
+        
+        return j == pushed.count
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题31. 栈的压入、弹出序列/Solution.swift

@@ -0,0 +1,16 @@
+class Solution {
+    func validateStackSequences(_ pushed: [Int], _ popped: [Int]) -> Bool {
+        var stack = [Int]()
+        var j = 0
+        
+        for v in pushed {
+            stack.append(v)
+            while !stack.isEmpty && stack.last == popped[j] {
+                stack.removeLast()
+                j += 1
+            }
+        }
+        
+        return j == pushed.count
+    }
+}