剑指 Offer 32 - III. 从上到下打印二叉树 III

• 时间复杂度
  O(n) 每个节点访问一次
• 空间复杂度
  O(n)

func levelOrder(_ root: TreeNode?) -> [[Int]] {
        guard let root = root else { return [] }
        var ans: [[Int]] = []
        var listTree: [TreeNode] = [root]

        var isLR = false
        while !listTree.isEmpty {
            var valList: [Int] = []
            var tempTree: [TreeNode] = []
            isLR = !isLR
            for tree in listTree {
                if tree.left != nil {
                    tempTree.append(tree.left!)
                }
                if tree.right != nil {
                    tempTree.append(tree.right!)
                }
                if isLR {
                    valList.append(tree.val)
                } else {
                    valList.insert(tree.val, at: 0)
                }
            }
            listTree = tempTree
            ans.append(valList)
        }
        return ans
    }