<leetcode> 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] 

算法： 同Combinations, DFS+recusion

首先对数组排序(题目要求升序输出)。

然后DFS+recusion进行枚举，题目要求每个元素可以使用多次，所以level指向当前的level而不是下一层，并在recusion中添加参数target，为当前的target-candidates[i](省去list求和过程)。返回条件是target＝＝0，添加当前list到result，或者target<0，当前组合不是solution，返回。

同Combination, 每次recusion传递前需要克隆一个list

public class Solution {    public int[] copy;    public List<List<Integer>> combinationSum(int[] candidates, int target) {        List<List<Integer>> result=new ArrayList<List<Integer>>();        ArrayList<Integer> solution=new ArrayList<Integer>();        if(candidates.length<=0){            return result;        }        else{        copy=new int[candidates.length];         mergeSort(candidates,0,candidates.length-1);            findCombination(candidates,target,0,result,solution);        }        return result;    }    public void findCombination(int[] candidates, int target, int level, List<List<Integer>> result, ArrayList<Integer> solution){        if(target==0){            result.add(solution);            return;        }        if(target<0){            return;        }        for(int i=level;i<candidates.length;i++){            ArrayList<Integer> solutionCopy=(ArrayList)solution.clone();//Clone list            solutionCopy.add(candidates[i]);            findCombination(candidates,target-candidates[i],i,result,solutionCopy);//level is current i(level)        }    }    /*    * Merge sort    */    public void mergeArray(int[] numbers,int low,int mid,int high){        int k=low,l=mid+1;        for(int i=low;i<=high;i++){            copy[i]=numbers[i];        }        for(int j=low;j<=high;j++){            if(k>mid){                numbers[j]=copy[l];                l++;            }            else if(l>high){                numbers[j]=copy[k];                k++;            }            else if(copy[k]<copy[l]){                numbers[j]=copy[k];                k++;            }            else if(copy[k]>=copy[l]){                numbers[j]=copy[l];                l++;            }        }    }    public void mergeSort(int[] numbers,int low,int high){        if(high<=low){            return;        }        int mid=(low+high)/2;        mergeSort(numbers,low,mid);        mergeSort(numbers,mid+1,high);        mergeArray(numbers,low,mid,high);    }}