https://leetcode.com/problems/symmetric-tree/ -- Definitely seems more like a medium

Leetcode: 101. Symmetric Tree

/**
 * 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
    }
    var helper func(left *TreeNode, right *TreeNode) bool
    helper = func(left *TreeNode, right *TreeNode) bool {
        if left == nil || right == nil {
            // both nil or wrong
            return left == nil && right == nil
        } else if left.Val != right.Val {
            return false
        }
        return helper(left.Left, right.Right) && helper(left.Right, right.Left)
    }
    return helper(root.Left, root.Right)
}