python 二叉树前序，中序，后序遍历 LeetCode No.144, No.94,No.145

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

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        return [] if root is None else [root.val] + self.preorderTraversal(root.left) +self.preorderTraversal(root.right)

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

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root is None:
            return []
        
        stack, output = [root], []
        
        while stack:
            root = stack.pop()
            if root is not None:
                output.append(root.val)
                if root.right is not None:
                    stack.append(root.right)
                if root.left is not None:
                    stack.append(root.left)
        
        return output

这题和98基本上是一模一样，所以写98题之后，顺便写了97题，刚好想用一行写完，就有了下面的写法。大家以后写的时候可以多试试，有很大的成就感，而且为枯燥的刷题生活，增添一点乐趣。今天暂时先写个中序遍历吧，等过几天在写个前序和中序，还有ass要做，哭ing。

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

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        return [] if root is None else self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)
        

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

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res,stack,p = [],[],root
        while p or stack:
            # 把左子树压入栈中
            while p:
                stack.append(p)
                p = p.left
            # 输出 栈顶元素
            tmp = stack.pop()
            res.append(tmp.val)
            # 看右子树
            p = tmp.right
        return res

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

class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        return [] if root is None else self.postorderTraversal(root.left) + self.postorderTraversal(root.right) +[root.val]
        

class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root is None:
            return []

        stack, output = [root ], []
        while stack:
            root = stack.pop()
            output.append(root.val)
            if root.left is not None:
                stack.append(root.left)
            if root.right is not None:
                stack.append(root.right)
                
        return output[::-1]