102. Binary Tree Level Order Traversal

层次遍历一个树,将结果按层分组放到列表中

class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        nodelist = [root]
        result = []
        while nodelist:
            tmplist = []
            vallist = []
            for node in nodelist:
                vallist.append(node.val)
                if node.left:
                    tmplist.append(node.left)
                if node.right:
                    tmplist.append(node.right)
            result.append(vallist)
            nodelist = tmplist
        return result