使用連結串列

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def Convert(self, pRootOfTree):
        # write code here
        if not pRootOfTree:
            return None
        l = []
        result = []
        l.append(pRootOfTree)
        while len(l):
            cur = l[-1]
            if cur.left:
                l.append(cur.left)
            elif not cur.left and not cur.right:
                result.append(cur)
                del l[-1]
                while len(l):
                    cur = l[-1]
                    result.append(cur)
                    del l[-1]
                    if cur.right:
                        l.append(cur.right)
                        break
            else:
                cur = l[-1]
                result.append(cur)
                del l[-1]
                l.append(cur.right)
        for i , j in enumerate(result[:-1]):
            j.right = result[i+1]
            result[i+1].left = j
        return result[0]
 

使用遞迴

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def Convert(self, pRootOfTree):
        # write code here
        if not pRootOfTree:
            return 
        self.result = []
        self.midTravel(pRootOfTree)
        for i , j in enumerate(self.result[:-1]):
            j.right = self.result[i+1]
            self.result[i+1].left = j
        return self.result[0]
        
        
    def midTravel(self,tree):
        if not tree:
            return
        self.midTravel(tree.left)
        self.result.append(tree)
        self.midTravel(tree.right)