单项选择题
The order of hierarchy traversal is:层次遍历的次序是:
A.The first root and then the left and the last right先根再左子最后右子
B.Left first and then right last right child先左子再根最后右子
C.Top-down access to each node in depth, from left to right in nodes of the same depth自上而下访问各个深度的节点,同样深度的节点中自左向右
D.Top-down access to each node in depth, from right to left in nodes of the same depth自下而上访问各个深度的节点,同样深度的节点中自左向右