99恢复二叉树

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
# 如果第一个节点没有找到，第一个就等于前一个节点大于root节点的前一个节点
# 如果第一个节点已经找到，第二个就等于前一个节点大于root节点的root节点
class Solution:
    def recoverTree(self, root: TreeNode) -> None:
        """
        Do not return anything, modify root in-place instead.
        """
        self.firstNode = None
        self.secondNode = None
        self.preNode = TreeNode(float("-inf"))
        self.dfs(root)
        self.firstNode.val,self.secondNode.val = self.secondNode.val,self.firstNode.val
    def dfs(self,root):
        if not root:
            return
        self.dfs(root.left)
        if self.firstNode == None and self.preNode.val > root.val:
            self.firstNode = self.preNode
        if self.firstNode and self.preNode.val > root.val:
            self.secondNode = root
        self.preNode = root
        self.dfs(root.right)