给一个target number，在BST上找到closest point

void Closed(TreeNode * root, const int val, int & res)
{
    if(root==NULL)
        return INT_MAX;
    if(root->left)
        Closed(root->left,val,res);
    if(abs(root->val - val) < abs(res - val))
    {
        res = root->val;
    }
    if(root->val < val && root->right)
    {
        Colsed(root->right,val,res);
    }
}