18. 4Sum

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> ret=new ArrayList<List<Integer>>();
        Arrays.sort(nums);
        
        for(int i=0;i<nums.length;i++)
            if(i==0||nums[i]!=nums[i-1])
                for(int j=i+1;j<nums.length;j++)
                    if(j==i+1||nums[j]!=nums[j-1])
                    {
                        int l=j+1;
                        int r=nums.length-1;
                        while(l<r)
                        {
                            int sum=nums[i]+nums[j]+nums[l]+nums[r];
                            if(sum==target)
                            {
                                ret.add(Arrays.asList(new Integer[]{nums[i],nums[j],nums[l],nums[r]}));
                                while(l<r&&nums[l]==nums[l+1])l++;
                                while(l<r&&nums[r]==nums[r-1])r--;
                                l++;
                                r--;
                            }
                            else if (sum<target)
                                l++;
                            else
                                r--;
                        }
                    }
        
        return ret;
    }
}