i just according to the catalan Formula,and write the code to get the answer,but it's wrong,when the input is 19，it looks like a Integer overflow error，but i didn't know why the dp method dont have such an error?

here is my code:

```
calss Solution{
public:
int numTrees(int n){
if(n==0) return 1;
else return numtrees(n-1)**(4**n-2)/(n+1);
}
};
```