16. Binary Tree Level Order Traversal

func levelOrderTraversal(root *TreeNode) [][]int {
    if root == nil {
        return nil
    }
    result := [][]int{}
    queue := []*TreeNode{root}

    for len(queue) > 0 {
        levelSize := len(queue)
        level := []int{}
        for i := 0; i < levelSize; i++ {
            node := queue[0]
            queue = queue[1:]
            level = append(level, node.Val)
            if node.Left != nil {
                queue = append(queue, node.Left)
            }
            if node.Right != nil {
                queue = append(queue, node.Right)
            }
        }
        result = append(result, level)
    }
    return result
}