LeetCode 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, … , a_k) must be in non-descending order. (ie, a₁ ≤ a₂ ≤ … ≤ a_k).
• 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]

 

求候选元素中所有和为目标值的组合，

候选元素可以多次取，组合不能完全相同。

虽然没有说明，实际所有的候选元素都是不同的。

递归求解。

 

代码：

class Solution {
	vector<vector<int>> ans;	//结果
	vector<int> temp;	//单次结果
public:
	void combs(vector<int> &cand,int target,int sum,int begin)	//候选集，目标，当前集合的和，当前集合用到的最大序号
	{
		if(sum==target)	//符合要求
		{
			ans.push_back(temp);
			return;
		}
		for(int i=begin;i<cand.size();i++)	//寻找
		{
			if(sum+cand[i]<=target)
			{
				temp.push_back(cand[i]);
				combs(cand,target,sum+cand[i],i);
				temp.pop_back();
			}
			else
				break;
		}
	}
    vector<vector<int> > combinationSum(vector<int> &candidates, int target) {
        ans.clear();
		temp.clear();
		if(candidates.empty())
			return ans;
		sort(candidates.begin(),candidates.end());	//！！！需要保证候选集合没有重复元素
		combs(candidates,target,0,0);
		return ans;
    }
};