39. Combination Sum

为了避免前面选了 到后面选的时候 又选到了后面的前面 加一个position作为标记 循环从position开始

 

class Solution {
    List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        backtrack(candidates, target, 0, new ArrayList<>(), 0);
        return res;
    }
    
    public void backtrack(int[] candidates, int target, int sum, List<Integer> list, int position) {
        if(sum > target) return;
        if(sum == target) {
            res.add(new ArrayList<>(list));
        }
        for(int i = position; i < candidates.length; i++) {
            list.add(candidates[i]);
            sum += candidates[i];
            backtrack(candidates, target, sum, list, i);
            list.remove(list.size() - 1);
            sum -= candidates[i];
        }
    }
}