LeetCode-39 Combination Sum

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] 

思路：使用回溯和递归

List<List<Integer>> lists = new ArrayList<List<Integer>>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        Arrays.sort(candidates);
        if(target == 0 || candidates == null || candidates.length == 0) {
            return lists;
        }
        combinationSumHelper(candidates, target, 0, new ArrayList<Integer>());
        return lists;
    }
    
    public void combinationSumHelper(int[] candidates, int target, int start, List<Integer> list) {
        for(int i=start; i<candidates.length; i++) {
            if(target > candidates[i]) {
                list.add(candidates[i]);
                combinationSumHelper(candidates, target-candidates[i], i, list);
                list.remove(list.size()-1);
            } else if(target == candidates[i]) {
                list.add(candidates[i]);
                lists.add(new ArrayList(list));
                list.remove(list.size()-1);
                return;
            } else {
                return;
            }
        }
    }