Efficient algorithm to compute combinations with repetitions of an array adding up to given sum

Question

So in a personal C++ project I am faced with a problem. I am rephrasing it as follows :

Given an array of n elements (e.g. [1, 3, 5], with n = 3 elements) where the number at the ith position denotes how many possible values the number at ith index can take (e.g here the first element can take 1 value which is 0; the second element can take 3 values from among 0,1,2; the third element can take 5 values from among 0,1,2,3,4).

I need to list all possible such arrays of length n that sum up to less than or equal to a given number k. Here is an example :

Input 1:

input array = [2,2]; k = 2

Output 1:

[0,0], [0,1], [1,0], [1,1]

Also, for instance :

Input 2:

input array = [2,2]; k = 1

Output 2:

[0,0], [0,1], [1,0]

The issue :

I have coded up a simple recursive and a simple iterative solution, which enumerates all arrays and only keeps those which have sum less than k. The problem with these is that for the case where n is large and k = 1, my code takes very long to run, since it enumerates all the cases and keeps a few.

I cannot see any overlapping sub-problems so I feel DP and memoization isn't applicable. How can I write the required C++ code for this that works?

Here is my code for the iterative version :

// enumerates all arrays which sum up to k

vector<vector<int> > count_all_arrays(vector<int> input_array, int k){

    vector<vector<int> > arr;
    int n = (int)input_array.size();

    // make auxilliary array with elements

    for(int i = 0; i < n; i++){
        vector<int> temp(input_array[i]);
        std::iota(temp.begin(), temp.end(), 0);
        arr.push_back(temp);
    }

    // computes combinations

    vector<int> temp(n);
    vector<vector<int> > answers;
    vector<int> indices(n, 0);
    int next;

    while(1){ 
        temp.clear();
        for (int i = 0; i < n; i++) 
            temp.push_back(arr[i][indices[i]]);  
        long long int total = accumulate(temp.begin(), temp.end(), 0);
        if(total <= k)
            answers.push_back(temp);
        next = n - 1; 
        while (next >= 0 &&  
              (indices[next] + 1 >= (int)arr[next].size())) 
            next--; 
        if (next < 0) 
            break; 
        indices[next]++; 
        for (int i = next + 1; i < n; i++) 
            indices[i] = 0; 
    }
    return answers;
}

Answer 1

It's a pretty simple recursive task:

#include <bits/stdc++.h>    
using namespace std;

int arr[] = {2, 2};
int n = 2;
int k = 2;

void gen(int pos, int sum, string s){
    if(pos == n){
        cout<<"["<<s<<" ]"<<endl;
        return;
    }
    for(int i = 0; i < arr[pos]; i++){
        if(sum + i > k) return;
        gen(pos + 1, sum + i, s + " " + to_string(i));
    }
}

int main(){
    gen(0, 0, "");
    return 0;
}

Just generate all possibilities for each slot of the array and for each choice, take the sum to the evaluation of the next slot.

When n is large and k = 1, it's natural that it takes O(n), since you will have:

[0, 0, 0, ..., 0, 0, 1]
[0, 0, 0, ..., 0, 1, 0]
[0, 0, 0, ..., 1, 0, 0]
          ...
[0, 0, 1, ..., 0, 0, 0]
[0, 1, 0, ..., 0, 0, 0]
[1, 0, 0, ..., 0, 0, 0]