Algorithm

Use backtracking to explore all possible number combinations that sum up to a target.
Track the current combination and remaining target as you build solutions incrementally.
If a valid combination meets both the target sum and required size, store it in the results.
Prune the path early if the sum goes negative or the combination exceeds the allowed size.
Explore the search space by trying each number, then undoing the choice to backtrack and try alternatives.

Code

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

class Solution {
private: 
    // Recursive function to find combinations
    void func(int sum, int last, vector<int> &nums, int k, vector<vector<int>> &ans) {
        // If the sum is zero and the number of elements is k
        if(sum == 0 && nums.size() == k) {
            // Add the current combination to the answer
            ans.push_back(nums);
            return; 
        }
        // If the sum is less than or equal to zero or the number of elements exceeds k
        if(sum <= 0 || nums.size() > k) return; 

        // Iterate from the last number to 9
        for(int i = last; i <= 9; i++) {
            // If the curren