Leetcode Problem 39 Combination Sum.txt

39. Combination Sum
Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

The same repeated number may be chosen from candidates unlimited number of times.

Note:

All numbers (including target) will be positive integers.
The solution set must not contain duplicate combinations.
Example 1:

Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
  [7],
  [2,2,3]
]
Example 2:

Input: candidates = [2,3,5], target = 8,
A solution set is:
[
  [2,2,2,2],
  [2,3,3],
  [3,5]
]

HINT : Use Backtracking. 
Make recursive calls for same index if repeated element is allowed or 
next index i+1 if no repetition of element is allowed.

public class Solution {
    public IList<IList<int>> CombinationSum(int[] candidates, int target) 
    {
        var result = new List<IList<int>>();
        Array.Sort(candidates);
        
        Backtrack( result,new List<int>(), candidates, target, 0);
        return result;
    }
    
    
    public void Backtrack(List<IList<int>> result, List<int> currentList, int[] candidateSorted, int target, int startIndex)
    {
        if(target < 0)
        {
            return;
        }
        else if(target == 0)
        {
            result.Add(new List<int>(currentList));
        }
        else
        {
            for(int i = startIndex; i< candidateSorted.Count(); ++i)
            {
                currentList.Add(candidateSorted[i]);
                Backtrack(result,currentList,candidateSorted, target - candidateSorted[i], i);
                currentList.RemoveAt(currentList.Count() - 1);
            }
        }
    }
}