leetcode998

 class Solution:

     def __init__(self):

         self.prelist = list()

     def preTree(self,root:TreeNode):

         if root != None:

             self.preTree(root.left)

             self.prelist.append(root.val)

             self.preTree(root.right)

     def Construct(self,ary):

         if len(ary)>0:

             result = TreeNode(ary[0])

             maxnum = ary[0]

             maxindex = 0

             for i in range(1,len(ary)):

                 if maxnum < ary[i]:

                     maxnum = ary[i]

                     maxindex = i

             result.val = maxnum

             result.left = self.Construct(ary[:maxindex])

             result.right = self.Construct(ary[maxindex+1:])

             return result

         else:

             return None

     def insertIntoMaxTree(self, root: TreeNode, val: int) -> TreeNode:

         prelist2 = list()

         self.preTree(root)

         prelist2 = self.prelist[:]

         prelist2.append(val)

         result = TreeNode(prelist2[0])

         maxnum = prelist2[0]

         maxindex = 0

         for i in range(1,len(prelist2)):

             if maxnum < prelist2[i]:

                 maxnum = prelist2[i]

                 maxindex = i

         result.val = maxnum

         result.left = self.Construct(prelist2[:maxindex])

         result.right = self.Construct(prelist2[maxindex+1:])

         return result

注意第3行，这里是用构造函数来初始化了一个“成员变量”，这样是可以正常工作的。但是如果使用“类变量”，如下面的第2行，在oj上就会报错。

 class Solution:

     prelist = list()

     def preTree(self,root:TreeNode):

         if root != None:

             self.preTree(root.left)

             self.prelist.append(root.val)

             self.preTree(root.right)

     def Construct(self,ary):

         if len(ary)>0:

             result = TreeNode(ary[0])

             maxnum = ary[0]

             maxindex = 0

             for i in range(1,len(ary)):

                 if maxnum < ary[i]:

                     maxnum = ary[i]

                     maxindex = i

             result.val = maxnum

             result.left = self.Construct(ary[:maxindex])

             result.right = self.Construct(ary[maxindex+1:])

             return result

         else:

             return None

     def insertIntoMaxTree(self, root: TreeNode, val: int) -> TreeNode:

         self.preTree(root)

         prelist2 = self.prelist[:]

         prelist2.append(val)

         result = TreeNode(prelist2[0])

         maxnum = prelist2[0]

         maxindex = 0

         for i in range(1,len(prelist2)):

             if maxnum < prelist2[i]:

                 maxnum = prelist2[i]

                 maxindex = i

         result.val = maxnum

         result.left = self.Construct(prelist2[:maxindex])

         result.right = self.Construct(prelist2[maxindex+1:])

         return result

为了解决这个问题，还可以使用一种方式，就是把结果集合传递到前序遍历的方法中，如下面第2行定义的方法，多了一个prelist参数，用于记录结果：

 class Solution:

     def preTree(self,root:TreeNode,prelist):

         if root != None:

             self.preTree(root.left,prelist)

             prelist.append(root.val)

             self.preTree(root.right,prelist)

     def Construct(self,ary):

         if len(ary)>0:

             result = TreeNode(ary[0])

             maxnum = ary[0]

             maxindex = 0

             for i in range(1,len(ary)):

                 if maxnum < ary[i]:

                     maxnum = ary[i]

                     maxindex = i

             result.val = maxnum

             result.left = self.Construct(ary[:maxindex])

             result.right = self.Construct(ary[maxindex+1:])

             return result

         else:

             return None

     def insertIntoMaxTree(self, root: TreeNode, val: int) -> TreeNode:

         prelist2 = list()

         self.preTree(root,prelist2)

         prelist2.append(val)

         result = TreeNode(prelist2[0])

         maxnum = prelist2[0]

         maxindex = 0

         for i in range(1,len(prelist2)):

             if maxnum < prelist2[i]:

                 maxnum = prelist2[i]

                 maxindex = i

         result.val = maxnum

         result.left = self.Construct(prelist2[:maxindex])

         result.right = self.Construct(prelist2[maxindex+1:])

         return result

总结：使用第1种写法最为稳妥。