Kth smallest element in a BST - C++ Code

Do inorder Traversal of BST and count number of elements traversed till now . As inorder traversal gives us the element in sorted Order, when count reaches k , we break out and Print the element.

/**
 * Definition for Binary Search Tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 * };
 */
int kthsmallest(TreeNode* root, int k) {
    if(root==NULL)
      return 0;
    stack<TreeNode*> s;
    bool complete=false;
    TreeNode* cur=root;
    int count=0;
    int val=0;
    while(!complete)
    {
        if(cur!=NULL)
        {
            s.push(cur);
            cur=cur->left;
        }
        else
        {
            if(!s.empty())
            {
                count++;
                cur=s.top();
                s.pop();
                if(count==k)
                {
                    val=cur->val;       
      break;
                }
                  cur=cur->right;
            }
            else
              complete=true;
        }  
    }
    return val;
}

Please comment if you find anything incorrect.