二叉树遍历

二叉树递归遍历

# 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 inorderTraversal(self, root: TreeNode) -> List[int]:
        # 前序
        perv = []
        # 中序
        mid = []
        # 后序
        last = []
        def find(root):
           if not root:
               return
           perv.append(root.val)
           find(root.left)
           mid.append(root.val)
           find(root.right)
           last.append(root.val)
        find(root)
        return [prev, mid, last]

二叉树非递归前序遍历（中左右）

# 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 postorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        res = []
        stack = [root]
        while stack:
            node = stack.pop()
            res.append(node.val)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return res

二叉树非递归中序遍历（左中右）

# 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 inorderTraversal(self, root: TreeNode) -> List[int]:
        stack = []
        res = []
        while root or stack:
            if root:
                stack.append(root)
                root = root.left
            else:
                node = stack.pop()
                res.append(node.val)
                root = node.right
        return res

二叉树非递归后序遍历（左右中 ==> 前序（中左右）> 中右左（反转））

# 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 postorderTraversal(self, root: TreeNode) -> List[int]:
        """
        后序等于前序（左右）的变换， 然后反转 得到
        """
        if not root:
            return []
        res = []
        stack = [root]
        while stack:
            node = stack.pop()
            res.append(node.val)
            if node.left:
                stack.append(node.left)
            if node.right:
                stack.append(node.right)
        # 翻转
        return res[::-1]

二叉树层序遍历

# 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 levelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        queue = [root]
        result = []
        while queue:
            level_vals = []
            for _ in range(len(queue)):
                node = queue.pop(0)
                level_vals.append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            result.append(level_vals)
        return result