题目：
题解：
func levelOrder(root *TreeNode) [][]int {
ret := [][]int{}
if root == nil {
return ret
}
q := []*TreeNode{root}
for i := 0; len(q) > 0; i++ {
ret = append(ret, []int{})
p := []*TreeNode{}
for j := 0; j < len(q); j++ {
node := q[j]
ret[i] = append(ret[i], node.Val)
if node.Left != nil {
p = append(p, node.Left)
}
if node.Right != nil {
p = append(p, node.Right)
}
}
q = p
}
return ret
}