代码随想录算法训练营第十三天｜二叉树的遍历（递归、迭代（未完全弄懂）、层次）

需要好好看一下迭代写法，层次遍历写了一道确实快速刷完了十道hhh

递归遍历

代码随想录

无脑递归hhh

# 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 preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        # 递归
        res = []
        def travesel(root: Optional[TreeNode]):
            if root == None:
                return
            res.append(root.val)
            travesel(root.left)
            travesel(root.right)
        travesel(root)
        return res
        

迭代遍历

代码随想录

只弄懂了前序遍历的迭代，需要好好看一下中绪、后序和统一的写法

# 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 preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        # 迭代
        stack = []
        res = []
        if root == None:
            return res
        stack.append(root)
        while len(stack) != 0:
            temp = stack[-1]
            stack.pop()
            res.append(temp.val)
            if  temp.right is not None:
                stack.append(temp.right)
            if temp.left is not None:
                stack.append(temp.left)
        return res

层次遍历

代码随想录

主要思路就是用队列保存每一层节点，出队列的时候，把孩子节点放入队列

# 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
from collections import deque
class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        mydeque = deque()
        mydeque1 = deque()
        levelRes = []
        res = []
        if root == None:
            return res
        mydeque.append(root)

        while len(mydeque) != 0 or len(mydeque1) != 0:
            temp = mydeque.popleft()
            levelRes.append(temp.val)
            if temp.left != None:
                mydeque1.append(temp.left)
            if temp.right != None:
                mydeque1.append(temp.right)
            if len(mydeque) == 0:
                res.append(levelRes)
                mydeque = mydeque1
                mydeque1 = deque()
                levelRes = []
        return res