Combination Sum

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] 

思路：

　　常见的回溯问题

我的代码：

public class Solution {

    public List<List<Integer>> combinationSum(int[] candidates, int target) {

        if(candidates == null || candidates.length == 0)    return rst;

        List<Integer> list = new ArrayList<Integer>();

        Arrays.sort(candidates);

        helper(list, candidates, target, 0, 0);

        return rst;

    }

    private List<List<Integer>> rst = new ArrayList<List<Integer>>();

    public void helper(List<Integer> list, int[] candidates, int target, int sum, int start)

    {

        if(sum > target)    return;

        if(sum == target)

        {

            if(!rst.contains(list))

                rst.add(new ArrayList(list));

            return;

        }

        for(int i = start ; i < candidates.length; i++)

        {

            list.add(candidates[i]);

            helper(list, candidates, target, sum + candidates[i], i);

            list.remove(list.size() - 1);

        }

    }

}

View Code