leetcode 39. Combination Sum

1.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]

 

2.思路 

从一堆数组中选取一些数，使得和等于target。与下一题不同的是一个数可以重复多次。

class Solution {
public:
void comb(vector<int> candidates, int index, int target, vector<vector<int>> &res, vector<int> &path)
{
if(0>target)return;
if(0==target){res.push_back(path);return;}
for(int i= index; i<candidates.size();i++)
{
path.push_back(candidates[i]);
comb(candidates,i,target-candidates[i],res,path);
path.pop_back();
}
}
vector<vector<int> > combinationSum(vector<int> &candidates, int target) {
sort(candidates.begin(),candidates.end());
vector<vector<int>> res;
vector<int> path;
comb(candidates,0,target,res,path);
return res;
}
};