求一个二叉搜索树中第K个最小值

std::stack<TreeNode*> Stack;
while (root || !Stack.empty())
{
if (root)
{
Stack.push(root);
root = root->left;
}
else
{
root = Stack.top();
Stack.pop();
if (--k == 0)
return root->val;
root = root->right;
}
}
}