leetcode : comobination sum [经典回溯]

Given a set of candidate numbers (C) (without duplicates) 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.
• 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 Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(candidates == null || candidates.length == 0) {
            return result;
        }
        List<Integer> list  = new ArrayList<Integer>();
        Arrays.sort(candidates);
        helper(result, list, candidates, target, 0);
        return result;
    }
    
    public void helper(List<List<Integer>> result, List<Integer> list, int[] nums, int remains, int position) {
        if(remains < 0) {
            return;
        }
        if(remains == 0) {
            result.add(new ArrayList<Integer>(list));
        }
        
        for(int i = position; i < nums.length; i++) {
            list.add(nums[i]);
            helper(result, list, nums, remains - nums[i],i);
            list.remove(list.size() - 1);
        }
    }
    
}