前序遍历法

按照前序遍历的框架，在前序遍历的编码区域处，直接交换左右节点。

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
# 
# @param pRoot TreeNode类 
# @return TreeNode类
#
class Solution:
    def Mirror(self , pRoot: TreeNode) -> TreeNode:
        if not pRoot: return 
        temp = pRoot.left
        pRoot.left = pRoot.right
        pRoot.right = temp
        # 前序遍历
        
        self.Mirror(pRoot.left)
        self.Mirror(pRoot.right)
        
        return pRoot