【力扣103. 二叉树的锯齿形层序遍历】BFS双端队列（多种解法）（python3）_力扣,层序遍历python3

题目描述

https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/

思路题解

自己写的方法，层序遍历后按层数逆转

class Solution:
    def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:return []
        from collections import deque
        ans=[[root.val]]
        stack=deque()
        stack.append(root)
        flag=0#右到左
        while stack:
            t=stack
            a=[]
            stack=deque()
            while t:
                node=t.popleft()
                if node.left:
                    stack.append(node.left)
                    a.append(node.left.val)
                if node.right:
                    stack.append(node.right)
                    a.append(node.right.val)
            if a:
                if flag:ans.append(a)
                else:ans.append(a[::-1])
            flag^=1
        return ans
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方法2：按层数头插或者尾插

https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/solution/er-cha-shu-de-ju-chi-xing-ceng-xu-bian-l-qsun/

class Solution:
    def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        from collections import deque
        if not root:
            return []
        q1,q2=deque(),deque()
        q1.append(root)
        flag=1
        ans=[]
        while q1:
            tmp=deque()
            while q1:
                node=q1.popleft()
                if flag==1:
                    tmp.append(node.val)
                else:
                    tmp.appendleft(node.val)
                if node.left:q2.append(node.left)
                if node.right:q2.append(node.right)
            flag=flag^1
            q1=q2.copy()
            q2.clear()
            ans.append(list(tmp))
        return ans
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