[Tree] Optimize solution to Binary Tree Zigzag Level Order Traversal

Tree/BinaryTreeZigzagLevelOrderTraversal.swift

@@ -22,33 +22,19 @@
 
 class BinaryTreeZigzagLevelOrderTraversal {
     func zigzagLevelOrder(root: TreeNode?) -> [[Int]] {
-        var res = [[Int]](), queue = [TreeNode](), isReverse = false
-
-        if let root = root {
-            queue.append(root)
+        guard let root = root else {
+            return [[Int]]()
         }
 
-        while !queue.isEmpty {
-            let size = queue.count
-            var level = [Int]()
-
-            for _ in 0..<size {
-                let node = queue.removeFirst()
-
-                // add val
-                level.insert(node.val, at: isReverse ? 0 : level.count) 
-
-                // add TreeNodes in next level
-                if let left = node.left {
-                    queue.append(left)
-                } 
-                if let right = node.right {
-                    queue.append(right)
-                }
-            }
+        var res = [[Int]](), isReverse = false, nodeQ = [root]
+
+        while !nodeQ.isEmpty {
+            let currentLevel = nodeQ.map { $0.val }
+            res.append(isReverse ? currentLevel.reversed() : currentLevel)
 
-            res.append(level)
             isReverse = !isReverse
+
+            nodeQ = nodeQ.flatMap { [$0.left, $0.right].compactMap { $0 } }
         }
 
         return res