遍历

class TreeNode:
    def __init__(self, value):
        self.value = value
        self.left = None
        self.right = None

def preorder_traversal(root):
    if root is not None:
        print(root.value, end=' ')
        preorder_traversal(root.left)
        preorder_traversal(root.right)

def inorder_traversal(root):
    if root is not None:
        inorder_traversal(root.left)
        print(root.value, end=' ')
        inorder_traversal(root.right)

def postorder_traversal(root):
    if root is not None:
        postorder_traversal(root.left)
        postorder_traversal(root.right)
        print(root.value, end=' ')

# 构建一个简单的二叉树
#     1
#    / \
#   2   3
#  / \
# 4   5
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)

# 执行遍历
print("前序遍历：", end='')
preorder_traversal(root)  # 输出: 1 2 4 5 3
print("\n中序遍历：", end='')
inorder_traversal(root)   # 输出: 4 2 5 1 3
print("\n后序遍历：", end='')
postorder_traversal(root) # 输出: 4 5 2 3 1