二叉搜索树的第K个节点

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
    int count = 0;
public:
    TreeNode* KthNode(TreeNode* pRoot, int k)
    {
        if(pRoot)
        {
           TreeNode* ret = KthNode(pRoot->left,k);
           if(ret)
                 return ret;
           if(++count==k)
                  return pRoot;
            ret = KthNode(pRoot->right,k);
            if(ret)
                  return ret;
        }
        return NULL;
    }
};