package main

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

func isSymmetric(root *TreeNode) bool {
	if root == nil {
		return true
	}
	var dfs func(lfNode, rfNode *TreeNode)
	flag := true
	dfs = func(lfNode, rfNode *TreeNode) {
		if lfNode != nil && rfNode != nil && lfNode.Val != rfNode.Val {
			flag = false
		}
		if !flag {
			return
		}
		if lfNode == nil || rfNode == nil {
			if lfNode != rfNode {
				flag = false
			}
			return
		}
		// 就是左结点按左根右遍历的结果和右结点按右根左遍历的结果相同...
		dfs(lfNode.Left, rfNode.Right)
		dfs(lfNode.Right, rfNode.Left)
	}
	dfs(root.Left, root.Right)
	return flag
}

func main() {

}