【代码随想录day14】二叉树的三种非递归遍历方式

先序遍历

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

中序遍历 

# 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 inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        if not root:
            return []
        stack = []
        cur = root
        res = []
        while cur or stack:
            if cur:
                stack.append(cur)
                cur = cur.left
            else:
                cur = stack.pop()
                res.append(cur.val)
                cur = cur.right
            
        return res

后序遍历 

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