Talk is cheap.

func isSymmetric(root *TreeNode) bool {
    if root == nil { return true }
    return isSym(root.Left, root.Right)

}

func isSym(p, q *TreeNode) bool {
    if p != nil && q != nil {
        return p.Val == q.Val && isSym(p.Left, q.Right) && isSym(p.Right, q.Left)
    }
    return p == q
}