N一ary tree level order traverse.
每个node有virtual depth。需要level order按照virtual dept输出node.
比如:
root(virtual depth 0)
/. |. ... .\
n2(1). |. n4(1)
/. \. |. /. \
n5(2). n6(2).n3(2). n7(2)
输出结果root, n2, n4, n5, n6, n3, n7