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

Each number in candidates may only be used once in the combination.

Note:

• All numbers (including target) will be positive integers.
• The solution set must not contain duplicate combinations.

Example 1:

Input: candidates = [10,1,2,7,6,1,5], target = 8,
A solution set is:
[
  [1, 7],
  [1, 2, 5],
  [2, 6],
  [1, 1, 6]
]

Example 2:

Input: candidates = [2,5,2,1,2], target = 5,
A solution set is:
[
  [1,2,2],
  [5]
]

思路：回溯法

 1 class Solution {
 2 public:
 3     vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
 4         int len = candidates.size();
 5         if (len == 0)
 6             return {};
 7         vector<vector<int> > res;
 8         vector<int> v;
 9         sort(candidates.begin(), candidates.end());
10         combinationSum2(candidates, target, res, v, len, 0);
11         return res;
12     }
13 private:
14     void combinationSum2(vector<int> &candidates, int target, vector<vector<int> > &res, vector<int> &v, int len, int begin) {
15         if (target == 0) {
16             res.push_back(v);
17             return ;
18         }
19         for (int i = begin; i < len && target >= candidates[i]; i++) {
20             if (i == begin || candidates[i] != candidates[i - 1]) {
21                 v.push_back(candidates[i]);
22                 combinationSum2(candidates, target - candidates[i], res, v, len, i + 1);
23                 v.pop_back();
24             }
25         }
26     }
27 };