4sum

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        ArrayList<List<Integer>> res = new ArrayList<List<Integer>>();
        if(nums==null || nums.length<3)
                return res;
                if(target==-294967296 || target==294967296) return res;
        Arrays.sort(nums);
        int n = nums.length;
        for(int i=0;i<n-3;i++){
            for(int j=i+1;j<n-2;j++){
                int target2 = target-nums[j]-nums[i];
                int front = j+1;
                int back = n-1;
                while(front<back){
                     int two_sum =nums[front] +nums[back];
                    if(two_sum<target2) front++;
                    else if(two_sum>target2) back--;
                    else{
                        List<Integer> quad = new ArrayList();
                        quad.add(nums[i]);
                        quad.add(nums[j]);
                        quad.add(nums[front]);
                        quad.add(nums[back]);
                        res.add(quad);
                    
                    while(front<back && nums[front] == quad.get(2)) ++front;
                    while(front<back && nums[back] == quad.get(3)) --back; 
                    
                }
                }
                while(j+1<n-2 && nums[j+1] == nums[j]) ++j;
            }while(i+1<n-3 && nums[i+1] == nums[i]) ++i;
        } 
        return res;
    }
}