combinationSum.cpp

/*Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where 
the chosen numbers sum to target. You may return the combinations in any order.The same number may be chosen from candidates an unlimited 
number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different.It is guaranteed that 
the number of unique combinations that sum up to target is less than 150 combinations for the given input.
Example 1:
Input: candidates = [2,3,6,7], target = 7
Output: [[2,2,3],[7]]
Explanation:
2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times.
7 is a candidate, and 7 = 7.
These are the only two combinations. */



class Solution {
public: 
    void findCombination(int ind, int target, vector<int> &arr, vector<vector<int>> &ans, vector<int>&ds) {
        if(ind == arr.size()) {
            if(target == 0) {
                ans.push_back(ds); 
            }
            return; 
        }
        // pick up the element 
        if(arr[ind] <= target) {
            ds.push_back(arr[ind]); 
            findCombination(ind, target - arr[ind], arr, ans, ds); 
            ds.pop_back(); 
        }
        
        findCombination(ind+1, target, arr, ans, ds); 
        
    }
public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        vector<vector<int>> ans; 
        vector<int> ds; 
        findCombination(0, target, candidates, ans, ds); 
        return ans; 
    }
};