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
}