102. 二叉树的层序遍历

• 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 treeList: [TreeNode] = [root]
        var ans: [[Int]] = []

        while !treeList.isEmpty {
            var valList: [Int] = []
            var tempList: [TreeNode] = []
            for tree in treeList {
                valList.append(tree.val)
                if tree.left != nil {
                    tempList.append(tree.left!)
                }
                if tree.right != nil {
                    tempList.append(tree.right!)
                }
            }
            ans.append(valList)
            treeList = tempList
        }
        return ans
    }