101. 对称二叉树

    1
   / \
  2   2
 / \ / \
3  4 4  3

    1
   / \
  2   2
   \   \
   3    3

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

func isSymmetric(root *TreeNode) bool {
	if root == nil {
		return true
	}

	return recur(root.Left, root.Right)
}

func recur(left, right *TreeNode) bool {
	if left == nil && right == nil {
		return true
	}

	if left == nil || right == nil {
		return false
	}

	return left.Val == right.Val && recur(left.Left, right.Right) && recur(left.Right, right.Left)
}