四数之和

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> res = new ArrayList<>();
        Arrays.sort(nums);
        for(int i=0;i<nums.length-3;i++){//四数之和
            int target1 = target - nums[i];
            if((i>0&&nums[i]==nums[i-1])) continue;
            for(int j=i+1;j<nums.length-2;j++){//转为三数之和
                if((j>i+1&&nums[j]==nums[j-1])) continue;
                int l = j+1,r=nums.length-1;
                while (l < r){//两数之和模板
                    int sum = nums[j]+nums[l]+nums[r];
                    if(sum == target1){
                        res.add(Arrays.asList(nums[i],nums[j],nums[l],nums[r]));
                        while(l < r && nums[r] == nums[r-1]) r--;
                        while (l < r && nums[l] == nums[l+1]) l++;
                        l++;r--;
                    }
                    else if(sum < target1) l++;
                    else r--;
                }
            }
        }

    return res;
    }
}