Combination Sum IV (hw mny combinz is psible for given sum)

class Solution {
public:
    int combinationSum4(vector<int>& nums, int n) {
        int i,j;
        vector<unsigned long long int>a(n+1);
        //memset(a,0,sizeof(a));
        a[0]=1;
        for(i=1;i<=n;i++)
        {
            for(j=0;j<nums.size();j++)
            {
                if(nums[j]<=i)
                {
                    a[i]+=a[i-nums[j]];
                }
            }
        }
        return a[n];
    }
};