第k小的结点

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
    int kk=-1;
    TreeNode* ret;
    TreeNode* KthNode(TreeNode* pRoot, int k)
    {
        if(k==0) return NULL;
        kk=k;
        KthNode(pRoot);
        if(kk<=0) return ret;
        else return NULL;
    }
    
    void KthNode(TreeNode* pRoot)
    {
        if(pRoot==NULL||kk<0) return;
        if(pRoot->left!=NULL) KthNode(pRoot->left);
        kk--;
        if(kk==0) ret = pRoot;
        if(pRoot->right!=NULL) KthNode(pRoot->right);
    }
};