二叉树的四种遍历顺序

题目 102 二叉树的层序遍历

思路

用队列实现层序遍历。

代码

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        res = []  # 最终结果
        if root is None:
            return res

        queue = []  # 队列
        queue.append(root)
        while queue:
            queue_size = len(queue)  # 记录每一层的元素个数
            temp = []  # 存放每一层的元素
            for i in range(queue_size):
                cur_node = queue.pop(0)  # 记录当前队列弹出的节点
                temp.append(cur_node.val)
                if cur_node.left is not None:
                    queue.append(cur_node.left)
                if cur_node.right is not None:
                    queue.append(cur_node.right)
            res.append(temp)
        return res