【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);
}
}