LeetCode Binary Tree Inorder Traversal

struct TreeNode {
int val;
TreeNode *left;
TreeNode *right;
TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

vector<int> inorderTraversal(TreeNode *root) {
TreeNode *p = root,*q=NULL;
vector<int> ret;
if(p==NULL)
return ret;
stack<TreeNode*> stk;
while(p||!stk.empty())
{
while(p)
{
stk.push(p);
p = p->left;
}
if(!stk.empty())
q = stk.top();
stk.pop();
ret.push_back(q->val);
p = q->right;
}
return ret;
}