f(0)=0

f(n)=sum(f(i)*f(n-1-i)),i=0..n-1.

```
class Solution {
public:
int numTrees(int n) {
int* nums=new int[n+1];
for(int i=0;i<n+1;i++)
nums[i]=0;
nums[0]=1;
for(int i=1;i<n+1;i++)
for(int j=0;j<i;j++)
nums[i]+=nums[j]*nums[i-1-j];
return nums[n];
}
};
```