[leetcode][回溯] Combination Sum

题目：

Given a set of candidate numbers (C) 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.
• Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
• 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) {        vector<vector<int> > res;        sort(candidates.begin(), candidates.end());        vector<int> oneCombination;        combinationSumCore(candidates, 0, target, oneCombination, res);        return res;    }private:    void combinationSumCore(vector<int>& candidates, int start, int target, vector<int> oneCombination, vector<vector<int> > &res){        for(int i = start; i < candidates.size(); ++i){            oneCombination.push_back(candidates[i]);//当前值大于target，则它后面的值也肯定大于target，不可能组成target            if(candidates[i] >= target){                if(candidates[i] == target) res.push_back(oneCombination);                break;            }            combinationSumCore(candidates, i, target-candidates[i], oneCombination, res);            oneCombination.pop_back();        }    }};