【LeetCode】4Sum

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]]

这种是一类问题，即KSum问题

4Sum可以类比于3Sum，做法与3Sum一样。相当于在3Sum的基础上。再嵌套一层for循环。

public class test2 {
    public static void main(String[] args) {
        int[] array = {1,0,-1,0,2,-2};
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        res = fourSum(array,0);
        for (int i = 0; i < res.size(); i++) {
            System.out.print("[");
            for(int j =0;j<res.get(i).size();j++){
                System.out.print(res.get(i).get(j)+" ");
            }
            System.out.println("]");
        }
    }

    public static List<List<Integer>> fourSum(int[] nums,int target){
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        int len = nums.length;
        if(nums==null || len<4){
            return null;
        }
        Arrays.sort(nums);
        for(int i = 0;i<len-3;i++){
            for(int j=i+1;j<len-2;j++){
                int start = j+1;
                int end = len-1;
                while(start<end){
                    int temp = nums[i]+nums[j]+nums[start]+nums[end];
                    if(temp==target){
                        res.add(Arrays.asList(nums[i],nums[j],nums[start],nums[end]));
                        start++;
                        end--;
                    }else if(temp<target){
                        start++;
                    }else{
                        end--;
                    }
                }
            }
        }
        return res;
    }
    
}