Talk is cheap.

func printFromTopToBottom(root *TreeNode) [][]int {
    res := [][]int{}
    if root != nil {
        level := []*TreeNode{root}
        for len(level) != 0 {
            restmp := []int{}
            for _, v := range level {
                restmp = append(restmp, v.Val)
            }
            res = append(res, restmp)
            tmp := []*TreeNode{}
            for _, v := range level {
                if v.Left != nil {
                    tmp = append(tmp, v.Left)
                }
                if v.Right != nil {
                    tmp = append(tmp, v.Right)
                }
            }
            level = tmp
        }
    }
    return res
}