LeetCode Unique Binary Search Trees

//  12ms
 1 class Solution {
public:
    int numTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int *f=new int[n+1];
        int i,j,sum;
        f[0]=f[1]=1;
        if(n<2)
            return 1;
        for(i=2;i<=n;i++)
        {
            sum=0;
            for(j=1;j<=i;j++)
                sum+=f[j-1]*f[i-j];  
            f[i]=sum;
        }
            
        return f[n];    
    }
};