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。