day12

今天是day12
第一题为二叉树的层序遍历

"遍历长度法"
"借用队列，将root加入其中"
class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        if not root:
            return []
        queue = collections.deque([root])
        result = []
        while queue:
            level = []
            "遍历queue"
            for _ in range(len(queue)):
                "左弹出并将值加入level中"
                cur = queue.popleft()
                level.append(cur.val)
                "开始层序遍历"
                if cur.left:
                    queue.append(cur.left)
                if cur.right:
                    queue.append(cur.right)
            result.append(level)
        return result

"递归法暂且不知何意"
class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        levels = []
        self.helper(root,0,levels)
        return levels
    def helper(self,node,level,levels):
        if not node:
            return 
        if len(levels) == level:
            levels.append([])
        levels[level].append(node.val)
        self.helper(node.left,level+1,levels)
        self.helper(node.right,level+1,levels)