力扣39.组合总数

输入: candidates = [2,3,6,7], target = 7,
所求解集为:
[
  [7],
  [2,2,3]
]

输入: candidates = [2,3,5], target = 8,
所求解集为:
[
  [2,2,2,2],
  [2,3,3],
  [3,5]
]

 1 class Solution {
 2     public List<List<Integer>> combinationSum(int[] candidates, int target) {
 3         List<List<Integer>> res = new ArrayList<>();
 4         LinkedList<Integer> temp = new LinkedList<>();
 5 
 6         traceback(candidates, 0, 0, target, temp, res);
 7         return res;
 8     }
 9 
10     public void traceback(int[] arr, int index, int nowSum, int target, LinkedList<Integer>temp, List<List<Integer>> res){
11         // 如果index >= 数组长度，判断是否满足要求
12         if(index >= arr.length){
13             if(nowSum == target){
14                 res.add(new LinkedList(temp));
15             }
16             return;
17         }
18         // 如果nowSum > target, 直接返回
19         if(nowSum > target){
20             return;
21         }
22 
23         // 不选择
24        traceback(arr, index + 1, nowSum, target, temp, res);
25 
26         // 选择该数,因为可以重复选择，所以index不加1
27         if(nowSum + arr[index] <= target){　　　　// 剪枝
28             temp.addLast(arr[index]);
29             traceback(arr, index, nowSum + arr[index], target,temp, res);
30             temp.removeLast();  // 回溯
31         }
32     }
33 }