获取中序序列

class Solution {
public:
    void rec(TreeNode* pRoot, vector<TreeNode*> &ret){
        if(pRoot!=NULL){
            rec(pRoot->left, ret);
            ret.push_back(pRoot);
            rec(pRoot->right, ret);
        }
    }

    TreeNode* KthNode(TreeNode* pRoot, int k)
    {
        if(k<1||pRoot==NULL) return NULL;
        //将二叉搜索树按照中序遍历（有序序列）存储在vector中
        vector<TreeNode*> ret;
        rec(pRoot,ret);
        //判断&返回
        int num=ret.size();
        if(num>=k)
            return ret[k-1];
        else
            return NULL;
    }
};