Update 0454.四数相加II.md

problems/0454.四数相加II.md

@@ -125,11 +125,11 @@ class Solution {
 ```
 
 Python：
-
+（版本一） 使用字典
 ```python
 class Solution(object):
     def fourSumCount(self, nums1, nums2, nums3, nums4):
-        # use a dict to store the elements in nums1 and nums2 and their sum
+        # 使用字典存储nums1和nums2中的元素及其和
         hashmap = dict()
         for n1 in nums1:
             for n2 in nums2:
@@ -138,7 +138,7 @@ class Solution(object):
                 else:
                     hashmap[n1+n2] = 1
         
-        # if the -(a+b) exists in nums3 and nums4, we shall add the count
+        # 如果 -(n1+n2) 存在于nums3和nums4, 存入结果
         count = 0
         for n3 in nums3:
             for n4 in nums4:
@@ -149,20 +149,18 @@ class Solution(object):
  
 
 ```
-
+（版本二）使用 defaultdict
 ```python
+from collections import defaultdict 
 class Solution:
     def fourSumCount(self, nums1: list, nums2: list, nums3: list, nums4: list) -> int:
-        from collections import defaultdict # You may use normal dict instead.
         rec, cnt = defaultdict(lambda : 0), 0
-        # To store the summary of all the possible combinations of nums1 & nums2, together with their frequencies.
         for i in nums1:
             for j in nums2:
                 rec[i+j] += 1
-        # To add up the frequencies if the corresponding value occurs in the dictionary
         for i in nums3:
             for j in nums4:
-                cnt += rec.get(-(i+j), 0) # No matched key, return 0.
+                cnt += rec.get(-(i+j), 0) 
         return cnt
 ```