题目：

Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

解答：

bool isSameTree(TreeNode *p, TreeNode *q)
{
  if (!p && !q)
  {
    return true;
  } 
  if (!p && q || p && !q || p->val != q->val)
  {
    return false;
  }
  bool left = isSameTree(p->left, q->left);
  bool right = isSameTree(p->right, q->right);
  return left && right;
}