Leetcode: Recover Binary Search Tree @Python

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
 
class Solution:
    # @param root, a tree node
    # @return a tree node
    def recoverTree(self, root):
        if not root: return None
        pre=parent=f1=f2=None# dummy node
        foundfirst=False
        crt=root
        while crt:
            if crt.left:
                pre=crt.left
                while pre.right and pre.right!=crt:
                    pre=pre.right
                if not pre.right:
                    pre.right=crt
                    crt=crt.left
                else:
                    pre.right=None
                    if parent.val>crt.val:
                        if not foundfirst:
                            f1=parent
                            foundfirst=True
                        f2=crt
                    parent=crt
                    crt=crt.right
            else:
                if parent and parent.val>crt.val:
                    if not foundfirst:
                        f1=parent
                        foundfirst=True
                    f2=crt
                parent=crt
                crt=crt.right
        if f1 and f2:
            f1.val,f2.val=f2.val,f1.val
        return root