深度优先遍历：前序、中序、后序

前序：根节点→左子树→右子树

中序：左子树→根节点→右子树

后序：左子树→右子树→根节点

 

1.实现一个树节点

 1 class TreeNode:
 2     def __init__(self,val,left=None,right=None):
 3         self.value = val
 4         self.leftChild = left
 5         self.rightChild = right        
 6 
 7 """构建一颗简单的树"""
 8 node = TreeNode(1)
 9 node2 = TreeNode(10)
10 root = TreeNode(5,node,node2)