39-Combination Sum

【问题】

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] 

【分析】

1.考虑用DFS的方式去解决该问题

2.由于需要考虑集合内的记录有序，且无重复，在递归的时候设置一个level

【算法实现】

public class Solution {
    List<List<Integer>> res;
    List<Integer> temp;
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        res=new ArrayList<List<Integer>>();
        temp=new ArrayList<Integer>();
        Arrays.sort(candidates);                     //考虑集合内的元素有序
        getCombination(candidates,target,0,0);
        return res;
    }
    
    public void getCombination(int[] candidates,int target,int sum,int level) {
        if(sum>target)
            return;
        if(target==sum) {
            res.add(new ArrayList<Integer>(temp));
            return;
        }
        for(int i=level;i<candidates.length;i++) {
            sum+=candidates[i];
            temp.add(candidates[i]);
            getCombination(candidates,target,sum,i);
            temp.remove(temp.size()-1);
            sum-=candidates[i];
        }
    }
}