leetcode-03-二叉树的锯齿层次遍历

题目描述：

方法一：

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

class Solution:
    def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        stack = [root]
        ans = []
        while stack:
            tem_stack=[]
            tem_ans = []
            for i in stack:
                tem_ans.append(i.val)
                if i.left:
                    tem_stack.append(i.left)
                if i.right:
                    tem_stack.append(i.right)
            stack = tem_stack
            ans.append(tem_ans)

        for i in range(len(ans)):
            if i%2!=0:
                ans[i] = ans[i][::-1]
        return ans

另：

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

class Solution:
    def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        stack = [root]
        ans = []
        depth = 0
        while stack:
            tem_ans,tem_stack = [],[]
            for i in stack:
                tem_ans.append(i.val)
                if i.left:
                    tem_stack.append(i.left)
                if i.right:
                    tem_stack.append(i.right)
            if depth%2 == 0:
                ans.append(tem_ans)
            else:
                ans.append(tem_ans[::-1])
            stack = tem_stack
            depth += 1
        return ans

方法二：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 zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        res = []
        def helper(root, depth):
            if not root: return
            if len(res) == depth:
                res.append([])
            if depth % 2 == 0:
                res[depth].append(root.val)
            else:
                res[depth].insert(0, root.val)
            helper(root.left, depth + 1)
            helper(root.right, depth + 1)
        helper(root, 0)
        return res