4sum.java

public class Solution {
    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(num == null || num.length < 4){
            return result;
        }
        Arrays.sort(num);
        for(int i = 0; i < num.length - 3; i++){
            if(i != 0 && num[i] == num[i-1]){
                continue;
            }
            for(int j = i + 1; j < num.length - 2; j++){
                if(j != i+1 && num[j] == num[j-1]){
                    continue;
                }
                int left = j + 1;
                int right = num.length - 1;
                while(left < right){
                    int sum = num[i] + num[j] + num[left] + num[right];
                    if(sum < target){
                        left++;
                    }else if(sum > target){
                        right--;
                    }
                    else{
                        ArrayList<Integer> tmp = new ArrayList<Integer>();
                        tmp.add(num[i]);
                        tmp.add(num[j]);
                        tmp.add(num[left]);
                        tmp.add(num[right]);
                        result.add(tmp);
                        left++;
                        right--;
                        while(left < right && num[left] == num[left-1]){
                            left++;
                        }
                        while(left < right && num[right] == num[right+1]){
                            right--;
                        }
                    }
                }
            }
            
        }
       
        return result;
        
    }
}