[Stack] add Solution to Preorder Traversal and Simplify Path

Stack/PreorderTraversal.swift

@@ -0,0 +1,37 @@
+/**
+ * Question Link: https://leetcode.com/problems/binary-tree-preorder-traversal/
+ * Primary idea: Use a stack to help iterate the tree
+ * Time Complexity: O(n), Space Complexity: O(n)
+ *
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.left = nil
+ *         self.right = nil
+ *     }
+ * }
+ */
+
+class PreorderTraversal {
+    func preorderTraversal(root: TreeNode?) -> [Int] {
+        var res = [Int]()
+        var stack = [TreeNode]()
+        var node = root
+
+        while !stack.isEmpty || node != nil {
+            if node != nil {
+                res.append(node!.val)
+                stack.append(node!)
+                node = node!.left
+            } else {
+                node = stack.removeLast().right
+            }
+        }
+
+        return res
+    }
+}

Stack/SimplifyPath.swift

@@ -0,0 +1,36 @@
+/**
+ * Question Link: https://leetcode.com/problems/simplify-path/
+ * Primary idea: Use a stack, normal to push, .. to pop 
+ * Time Complexity: O(n), Space Complexity: O(n)
+ */
+
+class SimplifyPath {
+    func simplifyPath(path: String) -> String {
+        var res = ""
+        var stack = [String]()
+        let paths = path.characters.split {$0 == "/"}.map(String.init)
+
+        for path in paths {
+            var path = path.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceCharacterSet())
+
+            guard path != "." else {
+                continue
+            }
+
+            if path == ".."  {
+                if (stack.count > 0) {
+                    stack.removeLast()
+                }
+            } else if path.characters.count > 0 {
+                stack.append(path)
+            }
+        }
+
+        for str in stack {
+            res += "/"
+            res += str
+        }
+
+        return res.isEmpty ? "/" : res
+    }
+}