a+b+c+d=target

Solution.java

public class Solution {
 public List<List<Integer>> fourSum(int[] num, int target) {
  ArrayList <List<Integer>> resultList = new ArrayList<>();
  int length = num.length;
  if (length < 4) 
   return resultList;
 
  Arrays.sort(num);
 
  for (int i = 0; i < length - 3; i++) {
   if (i > 0 && num[i] == num[i - 1]) continue;

   for (int j = i + 1; j < length - 2; j++) {
    if (j > i + 1 && num[j] == num[j - 1]) continue;
    int low = j + 1, high = length - 1;

    while (low < high) {
     int sum = num[i] + num[j] + num[low] + num[high];

     if (sum == target) {
      List<Integer> result = Arrays.asList(num[i], num[j], num[low], num[high]);
      resultList.add(result);

      while (num[high] == num[high - 1] && low < high)
          high = high - 1;

      while (num[low] == num[low + 1] && low < high)
          low = low + 1;

      high = high - 1;
      low = low + 1;
     } else if (sum > target) 
         high = high - 1;
     else 
         low = low + 1;
    }
   }
  }
  return resultList;
 }
}