【leetcode】【39】Combination Sum

一、问题描述

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] 

二、问题分析

排列组合类的题目均属于backtracking，可以参考这里。

三、Java AC代码

public List<List<Integer>> combinationSum(int[] candidates, int target) {List<List<Integer>> list = new ArrayList<List<Integer>>();List<Integer> item = new ArrayList<Integer>();if (candidates==null || candidates.length==0 || target<=0) {return list;}Arrays.sort(candidates);dfsHelper(list, item, candidates, target, 0, 0);return list;}public void dfsHelper(List<List<Integer>> list, List<Integer> item, int[] candidates, int target, int sum, int start){if (sum==target) {list.add(new ArrayList<Integer>(item));return ;}if (sum>target) {return ;}for (int i = start; i < candidates.length; i++) {sum += candidates[i];item.add(candidates[i]);dfsHelper(list, item, candidates, target, sum, i);sum -= candidates[i];item.remove(item.size()-1);}}