40.Combination Sum II

思路：

• 类似Combination Sum，但是数组有重复元素，同时要求元素不能重复使用。复杂度和Combination Sum类似。

class Solution {
public:
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
        vector<vector<int>> res;
        vector<int> tmp;
        sort(begin(candidates),end(candidates));
        backtrace(res,tmp,candidates,0,target,0);
        return res;
    }
    void backtrace(vector<vector<int>>& res,vector<int>& tmp,vector<int>& candidates,int sum,int target,int start){
        if(sum > target) return ;
        if(sum == target) res.push_back(tmp);
        for(int i = start; i < candidates.size(); i++){
            if(i > start && candidates[i] == candidates[i-1]) continue;   //避免了重复的数组
            tmp.push_back(candidates[i]);
            backtrace(res,tmp,candidates,sum+candidates[i],target,i+1);   //i+1指元素不能重复使用
            tmp.pop_back();
        }
    }
};