leetCode-nSum

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note: The solution set must not contain duplicate quadruplets.

For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]

通用解决办法

public static List<List<Integer>> kSum(int[] nums, int target) {
    List<List<Integer>> result = new ArrayList<List<Integer>>();
    Arrays.sort(nums);
    result = recursionRoutin(nums,0,4,0);
    return result;
}

public static List<List<Integer>> recursionRoutin(int[] nums,int begin,int k,int target){
    HashSet<List<Integer>> elementSet = new HashSet<List<Integer>>();
    List<List<Integer>> result = new ArrayList<List<Integer>>();
    List<List<Integer>> subResult = new ArrayList<List<Integer>>();
    //Recursion Base
    if(k == 2){
        int left = begin;
        int right = nums.length - 1;
        while(left < right){
            int sum = nums[left] + nums[right];
            if(sum == target){
                List<Integer> taplet = new ArrayList<Integer>();
                taplet.add(nums[left]);
                taplet.add(nums[right]);
                //Avoid reduplication
                if(!elementSet.contains(taplet)){
                    result.add(taplet);
                    elementSet.add(taplet);
                }
                left ++;
                right --;
            }else if(sum < target){
                left ++;
            }else{
                right --;
            }
        }
        return result;
    }else{
        for(int i = begin;i < nums.length - k - 1;i ++){
            subResult = recursionRoutin(nums,i + 1,k - 1,target - nums[i]);
            //System.out.println(k + " " + subResult);
            if (!subResult.isEmpty()) {
                for (int j = 0; j < subResult.size(); j++) {
                    subResult.get(j).add(nums[i]);
                    result.add(subResult.get(j));
                }
            }
        }
    }
    return result;
}