Leetcode 39. Combination Sum

39. Combination Sum

• Total Accepted: 102477
• Total Submissions: 316416
• Difficulty: Medium

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.
• 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：回溯法遍历元素，满足条件就返回。

代码：

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