【leetcode】230. 二叉搜索树中第K小的元素

int recursion(struct TreeNode* root, int* k,int* ret){
    if(!root || *k==0)
        return *ret;
    recursion(root->left,k,ret);    
    if(--(*k)==0){
        *ret = root->val;
    }
    recursion(root->right,k,ret);
    return *ret;    
}
int kthSmallest(struct TreeNode* root, int k){
    int ret=0;
    return recursion(root,&k,&ret);
}