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> combinationSum(vector& candidates, int target) {
        //Don't disturb the original array
        vector can(candidates.begin(), candidates.end());
        
        //In this situation, we don't need to sort the array
        //sort(can.begin(), can.end());
        vector> ret;
        vector tmp;
        bt(can, tmp, target, 0, ret);
        return ret;
    }
private:
    
    void bt(vector &can, vector &tmp, int remain, int start, vector> &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。