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// @Title: 对称二叉树 (Symmetric Tree)
// @Author: 15816537946@163.com
// @Date: 2020-08-30 00:36:27
// @Runtime: 4 ms
// @Memory: 2.8 MB
/**
 * 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
	}
	return check(root.Left, root.Right)
}

func check(left, right *TreeNode) bool {
	if left == nil && right == nil {
		return true
	}
	if left == nil || right == nil {
		return false
	}
	return left.Val == right.Val && check(left.Left, right.Right) && check(left.Right, right.Left)
}