18. 4Sum

//O(n3) time and O(1) space
class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> list = new ArrayList();
        //Sanity Check
        if(nums==null || nums.length<4) return list;
        int n = nums.length;
        Arrays.sort(nums);
        for(int i=0;i<n-3;i++){
            if(i>0 && nums[i]==nums[i-1]) continue;
            for(int j=i+1;j<n-2;j++){
                if(j>i+1 && nums[j]==nums[j-1]) continue;
                int low = j+1, high = n-1;
                while(low<high){
                    int sum = nums[i] + nums[j] + nums[low] + nums[high];
                    if(sum==target){
                        List<Integer> sublist = new ArrayList();
                        sublist.add(nums[i]);
                        sublist.add(nums[j]);
                        sublist.add(nums[low]);
                        sublist.add(nums[high]);
                        list.add(sublist);
                        while(low<high && nums[low]==nums[low+1]) low++;  
                        while(low<high && nums[high]==nums[high-1]) high--;
                        low++;
                        high--;
                    }
                    else if(sum<target){
                        low++;
                    }
                    else{
                        high--;
                    }
                }
            }
        }
        return list;
    }
}