二叉树层序遍历 通解bfs 👌

//2，bfs常规解
func levelOrderBottom(root *TreeNode) [][]int {
    if root == nil {
        return nil
    }
    levels := [][]int{}
    queue := []*TreeNode{root}
    for len(queue) > 0 {
        n := len(queue)
        level_tmp := []int{}
        for i := 0; i < n; i++ {
            root = queue[0]
            queue = queue[1 :]
            level_tmp = append(level_tmp, root.Val)
            if root.Left != nil {
                queue = append(queue, root.Left) 
            }
            if root.Right != nil {
                queue = append(queue, root.Right)
            }
        }
        levels = append(levels, level_tmp)
    }
    return levels
}