題目：

題解：

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
}