package main

type TreeNode struct {
	Val   int
	Left  *TreeNode
	Right *TreeNode
}

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func isSymmetric(root *TreeNode) bool {
	if root == nil {
		return true
	}
	queue := []*TreeNode{root}
	for len(queue) != 0 {
		l := len(queue)
		current := queue
		queue = queue[l:]
		for idx, node := range current {
			if (node == nil && current[l-idx-1] != nil) ||
				(node != nil && current[l-idx-1] == nil) ||
				(node != nil && node.Val != current[l-idx-1].Val) {
				return false
			}
			if node != nil {
				queue = append(queue, node.Left, node.Right)
			}
		}
	}
	return true
}