diff --git a/problems/0018.四数之和.md b/problems/0018.四数之和.md
index bad258c1..348f2f73 100644
--- a/problems/0018.四数之和.md
+++ b/problems/0018.四数之和.md
@@ -212,6 +212,7 @@ class Solution(object):
         
         # good thing about using python is you can use set to drop duplicates.
         ans = set()
+        # ans = []  # save results by list()
         for i in range(len(nums)):
             for j in range(i + 1, len(nums)):
                 for k in range(j + 1, len(nums)):
@@ -220,10 +221,16 @@ class Solution(object):
                         # make sure no duplicates.
                         count = (nums[i] == val) + (nums[j] == val) + (nums[k] == val)
                         if hashmap[val] > count:
-                            ans.add(tuple(sorted([nums[i], nums[j], nums[k], val])))
-                    else:
-                        continue
-        return ans
+                          ans_tmp = tuple(sorted([nums[i], nums[j], nums[k], val]))
+                          ans.add(ans_tmp)
+                          # Avoiding duplication in list manner but it cause time complexity increases
+                          # if ans_tmp not in ans:  
+                          #     ans.append(ans_tmp)
+                        else:
+                            continue
+        return list(ans) 
+        # if used list() to save results, just 
+        # return ans
         
 ```