Combination Sum

LeetCode

Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

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

Note:

• All numbers (including target) will be positive integers.
• Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
• The solution set must not contain duplicate combinations.

For example, given candidate set 2,3,6,7 and target 7, 
A solution set is: 
[7] 
[2, 2, 3] 

Solution: DFS

calculate each possible combination

	/*
	Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.
	The same repeated number may be chosen from C unlimited number of times.
	Note:
	All numbers (including target) will be positive integers.
	Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
	The solution set must not contain duplicate combinations.
	For example, given candidate set 2,3,6,7 and target 7,
	A solution set is:
	[7]
	[2, 2, 3]
	*/
	public class CombinationSum {
	public ArrayList<ArrayList<Integer>> combinationSum(int[] candidates, int target) {
	ArrayList<ArrayList<Integer>> result=new ArrayList<ArrayList<Integer>>();
	if (candidates==null || candidates.length==0|| target<0){
	return result;
	}
	Arrays.sort(candidates);
	int start=0;
	ArrayList<Integer> current=new ArrayList<Integer>();
	buildResult(candidates, start, current, target, result);
	return result;
	}
	private void buildResult(int[] candidates, int start, ArrayList<Integer> current, int target, ArrayList<ArrayList<Integer>> result){
	if (target==0){
	ArrayList<Integer> temp=new ArrayList<Integer>(current);
	result.add(temp);
	return;
	}
	for (int i=start; i<candidates.length; i++){
	if (target-candidates[i]<0){
	return;
	}
	current.add(candidates[i]);
	buildResult(candidates, i,current, target-candidates[i], result );
	current.remove(current.size()-1);
	}
	}
	}

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