54二叉搜索树的第k个结点

class Solution {
public:
    int cnt=0;
    TreeNode* KthNode(TreeNode* pRoot, int k)
    {
        if(!pRoot)return NULL;
        TreeNode*node=KthNode(pRoot->left,k);
        if(node)return node;
        cnt++;
        if(k==cnt)return pRoot;
        node=KthNode(pRoot->right,k);
        if(node)return node;
    }
};