leetcode--39. Combination Sum

Given a set of candidate numbers (C) (without duplicates) 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.
• 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]]

class Solution {public:        vector<vector<int>> combinationSum(vector<int>& candidates, int target) {        //Don't disturb the original array        vector<int> can(candidates.begin(), candidates.end());                //In this situation, we don't need to sort the array        //sort(can.begin(), can.end());        vector<vector<int>> ret;        vector<int> tmp;        bt(can, tmp, target, 0, ret);        return ret;    }private:        void bt(vector<int> &can, vector<int> &tmp, int remain, int start, vector<vector<int>> &ret){        if(remain < 0) return;        if(remain == 0) ret.push_back(tmp);        for(int i = start; i < can.size(); i++) {            tmp.push_back(can[i]);            //here we input i, not i+1, because elements can be reuse            bt(can, tmp, remain-can[i], i, ret);            tmp.pop_back();//for backtrace        }        return;    }};

需要注意的是，这道题目在assess的时候，给出的数据集就是不包含重复数据的，所以代码中也无需考虑重复数据集的问题。但是第40题则需要考虑。

两个题目结合起来看。