LeetCode 38 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

public class Solution {public List<List<Integer>> combinationSum(int[] candidates, int target) {List<List<Integer>> result = new LinkedList<List<Integer>>();Arrays.sort(candidates);LinkedList<Integer> intermediate=new LinkedList<Integer>();dfs(candidates,target,0,intermediate,result);return result;}private void dfs(int[] candidates,int gap,int level,List<Integer> intermediate,List<List<Integer>> result){if(gap==0){result.add(new LinkedList<Integer>(intermediate));return;}for(int i=level;i<candidates.length;i++){if(gap<candidates[i]) return;intermediate.add(candidates[i]);dfs(candidates,gap-candidates[i],i,intermediate,result);intermediate.remove(intermediate.size()-1);}}}