树-102. 二叉树的层次遍历-PYTHON

迭代法：

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

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = list()
        if not root:
            return res
            
        stack = list()
        stack.append(root)
        level = 0

        while stack:
            res.append([])
            count = len(stack)
            for i in range(count):
                tmp = stack.pop(0)
                res[level].append(tmp.val)

                if tmp.left:
                    stack.append(tmp.left)
                if tmp.right:
                    stack.append(tmp.right)
            level += 1
        return res

递归法：

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

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = list()
        if not root:
            return res
        level = 0


        def helper(root,level):
            if len(res) == level: #这行代码是精髓，判断每一层的开始，细细品味
                res.append([])
            
            res[level].append(root.val)

            if root.left:
                helper(root.left,level+1)
            if root.right:
                helper(root.right,level+1)
        
        helper(root,0)
        return res

来源：CSDN

作者：宋建国

链接：https://blog.csdn.net/hot7732788/article/details/104518628