LeetCode-102、二叉树的层次遍历-中等

给定一个二叉树，返回其按层次遍历的节点值。 （即逐层地，从左到右访问所有节点）。

例如：
给定二叉树: [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回其层次遍历结果：

[
  [3],
  [9,20],
  [15,7]
]

二叉树的层次遍历

 

代码1：迭代

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        res = []
        stack = [root]
        while stack:
            cur = []
            nodes = []
            for tmp in stack:
                if tmp is None:
                    continue
                else:
                    cur.append(tmp.val)
                    nodes.append(tmp.left)
                    nodes.append(tmp.right)
            stack = nodes
            if cur:
                res.append(cur)
        return res

 

代码2：递归

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        res = []
        def recursion(cur, level):
            if len(res) == level:
                res.append([])
            res[level].append(cur.val)
            if cur.left:
                recursion(cur.left, level+1)
            if cur.right:
                recursion(cur.right, level+1)
        if root:
            recursion(root, 0)
        return res