Python 代码

class Solution:
    def convert(self, root):
        if not root:return
        res =[]
        self.dfs(root,res)
        for i,v in enumerate(res[:-1]):
            v.right = res[i + 1]
            res[i + 1].left = v
        return res[0]

    def dfs(self,root,res):
        if not root:return
        self.dfs(root.left,res)
        res.append(root)
        self.dfs(root.right,res)