二叉搜索树第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 {
public:
    TreeNode* KthNode(TreeNode* pRoot, int k)
    {
        if(!pRoot) return nullptr;
        vector<TreeNode*> res;
        Sortnode(pRoot,res);
        if(k<=0 || k>res.size()) return nullptr;
        
        return res[k-1];
    }
    void Sortnode(TreeNode* root,vector<TreeNode*>& vec){
        if(!root) return;
        Sortnode(root->left,vec);
        vec.push_back(root);
        Sortnode(root->right,vec);
    }
};