https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

binary-tree-level-order-traversal-ii.go

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */

type LevelNode struct {
    node *TreeNode
    level int
}

func levelOrderBottom(root *TreeNode) [][]int {
    order := [][]int{}
    
    if root == nil {
        return order
    }

    queue := []*LevelNode{{root, 0}}
    
    for len(queue) > 0 {
        item := queue[0]
        queue = queue[1:]
        
        if len(order) <= item.level {
            order = append(order, []int{item.node.Val})
        } else {
            order[item.level] = append(order[item.level], item.node.Val)
        }
        
        if item.node.Left != nil {
            queue = append(queue, &LevelNode{item.node.Left, item.level + 1})
        }
        
        if item.node.Right != nil {
            queue = append(queue, &LevelNode{item.node.Right, item.level + 1})
        }
    }
    
    return reverse(order)
}

func reverse(arr [][]int) [][]int {
    for i, j := 0, len(arr) - 1; i < j; i, j = i + 1, j - 1 {
        arr[i], arr[j] = arr[j], arr[i]
    }

    return arr
}