二叉树的前序、中序、后序遍历的递归和非递归实现

一、二叉树的前序遍历（LeetCode 144）

递归实现：

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

class Solution:
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        return [root.val] +self.preorderTraversal(root.left)+self.preorderTraversal(root.right)

非递归实现：

class Solution:
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res=[]
        stack=[root]
        while stack:
            node=stack.pop()
            if node:
                res.append(node.val)
                stack.append(node.right)
                stack.append(node.left)
        return res 

二、二叉树的中序遍历（LeetCode 94）

递归实现：

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

class Solution:
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        return self.inorderTraversal(root.left)+[root.val]+self.inorderTraversal(root.right)

非递归实现：

class Solution:
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res,stack=[],[]
        while True:
            while root:
                stack.append(root)
                root=root.left
            if not stack:
                return res
            node=stack.pop()
            res.append(node.val)
            root=node.right
        return res

三、二叉树的后序遍历（LeetCode 145） 

递归实现：

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

class Solution:
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """        
        if not root:
            return []
        return self.postorderTraversal(root.left)+self.postorderTraversal(root.right)+[root.val]

非递归实现：

class Solution:
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res=[]
        stack=[root]
        while stack:
            node=stack.pop()
            if node:
                res.append(node.val)
                stack.append(node.left)
                stack.append(node.right)
        return res[::-1]