'''
树的构建：
   3
 9   20
   15  7
'''


class Tree():
  '树的实现'
  def __init__(self,data,left = 0,right = 0):
    self.left = left
    self.right = right
    self.data = data

  def __str__(self):
    return str(self.data)
# test tree


tree1 = Tree(data=15)
tree2 = Tree(data=7)
tree3 = Tree(20,tree1,tree2)
tree4 = Tree(data=9)
base = Tree(3,tree4,tree3)

def function(root):
  A = []
  result = []
  if not root:
    return result
  A.append(root)
  while A:
    current_root = A.pop(0)
    result.append(current_root.data)
    if current_root.left:
      A.append(current_root.left)
    if current_root.right:
      A.append(current_root.right)
  print(result)
  return result

function(base)

[3, 9, 20, 15, 7]

Process finished with exit code 0