【面试】反转二叉树 binary tree

问题
任意给一个二叉树，使之左右镜像反转。

这是一个经典面试题，考察tree这种数据结构的构建和递归操作。

class Node(object):
  def __init__(self, val):
    self.val = val.astype(np.int32)
    self.left = None
    self.right = None

def create_subtree(n_layers, vals, layer):
  '''Inputs a list `vals` to provide the values for the tree nodes.
  It assumes that `vals` contains enough elements to fill all the Nodes of
  this tree, and takes the element in the middle of this list `vals` to 
  build the current Node.
  '''
  if layer >= n_layers:
    return None
  pivot = len(vals) // 2
  root = Node(vals[pivot])
  root.left = create_subtree(n_layers, vals[:pivot], layer + 1)
  root.right = create_subtree(n_layers, vals[pivot+1:], layer + 1)
  return root

def swap_subtree(node):
  if node is None: return
  node.left = swap_subtree(node.left)
  node.right = swap_subtree(node.right)
  node.right, node.left = node.left, node.right
  return node

def main():
  n_layers = 5
  vals = np.array(range(0, 100))
  bt = create_subtree(n_layers, vals, 0)
  bt_swap = swap_subtree(bt)

以上main()函数给出一个测试用例。测试结果的动画贴在本文的头部。