边工作边刷题：70天一遍leetcode: day 65

Invert Binary Tree

要点：这题很简单，一点点扩展是如果iterative做。pre-order还是post-order呢？根据recursion，因为既可以先reverse再递归，也可以先递归再reverse，所以没有区别。in-order可能也能用，不过会比较复杂。这题说明binary tree的题应该从从递归式想迭代的方法。

# 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 invertTree(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        if not root: return root
        stk = [root]
        while stk:
            top = stk.pop()
            top.left, top.right = top.right, top.left
            if top.right:
                stk.append(top.right)
            if top.left:
                stk.append(top.left)
        
        return root