18. 4Sum

• Question Description
  
• My Key

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Test {
     public static void main(String[] args) {
     }
   public static List<List<Integer>> fourSum(int[] nums, int target) {           
      List<List<Integer>>  answer = new ArrayList();
      int length = nums.length;
      Arrays.sort(nums);
      
      if (nums == null || length < 4) {
            return answer;
      }
      
      for (int i = 0; i < length-3; i++) {
            if (i > 0 && nums[i] == nums[i - 1])
                  continue;
            
            for (int j = i+1; j < nums.length-2; j++) {
                  if (j > i+1 && nums[j] == nums[j - 1])
                        continue;
                  
                  int left = j+1;
                  int right = nums.length - 1;
                  
                  while(left < right) {
                        int sum = nums[i] + nums[j] + nums[left] +  nums[right];
                        
                        if (sum == target) {
                              answer.add(Arrays.asList(nums[i], nums[j],  nums[left], nums[right]));
                              
                              while(left < right && nums[left] ==  nums[left+1])
                                    left++;
                              while(left < right && nums[right] ==  nums[right-1])
                                    right--;
                              left++;
                              right--;
                        } else if (sum > target) {
                              right--;
                        } else {
                              left++;
                        }
                  }
            }
      }
      
      return answer;
   }
}