Leetcode——39. Combination Sum

题目原址

https://leetcode.com/problems/combination-sum/description/

题目描述

Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.
The same repeated number may be chosen from candidates unlimited number of times.

Note:

• All numbers (including target) will be positive integers.
• The solution set must not contain duplicate combinations.

Example1：

Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
[7],
[2,2,3]
]

Example2：

Input: candidates = [2,3,5], target = 8,
A solution set is:
[
[2,2,2,2],
[2,3,3],
[3,5]
]

解题思路

该题与216题是一个类型题，当然解题思路也是一样。注意：当要求解的结果是一些列集合的集合的时候，要使用DFS搜索记录路径。

AC代码

class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<Integer> list = new ArrayList<Integer>();
        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        Set<List<Integer>> ret1 = new HashSet<List<Integer>>();
        List<List<Integer>> ret2 = new ArrayList<List<Integer>>();
        re(candidates, ret, list, 0, 0, target);

        return ret;
    }

    public void re(int[] candidates, List<List<Integer>> ret, List<Integer> list, int sum, int k, int target) {
        if(sum == target) {
            if(!ret.contains(list))
                ret.add(new ArrayList(list));
            return;
        }
        else if(sum > target) return;

        for(int i = k; i < candidates.length; i++) {
            list.add(candidates[i]);

            re(candidates, ret, list, sum + candidates[i], i, target);
            list.remove(list.size() - 1);
        }
    }

}