leetcode 038 —— Combination Sum

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]

 

思路：回溯法，注意一定要先排序

class Solution {
public:
vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
vector<vector<int>> res;
vector<int> path;
int sum=0;
sort(candidates.begin(), candidates.end());
scan(0, sum, target, path, candidates, res);
return res;
}
//path使用& 显著提升速度
void scan(int level, int &sum, int &target, vector<int> &path, vector<int>& candidates,vector<vector<int>> & res){
if (sum > target)
return;
if (sum == target){
res.push_back(path);
return;
}
for (int i = level; i < candidates.size(); i++){
sum += candidates[i];
path.push_back(candidates[i]);
scan(i, sum, target, path, candidates, res);
sum -= candidates[i];
path.pop_back();
}
}
};