add python solution to 0018.4sum

problems/0018.四数之和.md

@@ -165,7 +165,39 @@ class Solution {
 ```
 
 Python：
+```python3
 
+class Solution(object):
+    def fourSum(self, nums, target):
+        """
+        :type nums: List[int]
+        :type target: int
+        :rtype: List[List[int]]
+        """
+        # use a dict to store value:showtimes
+        hashmap = dict()
+        for n in nums:
+            if n in hashmap:
+                hashmap[n] += 1
+            else: 
+                hashmap[n] = 1
+        
+        # good thing about using python is you can use set to drop duplicates.
+        ans = set()
+        for i in range(len(nums)):
+            for j in range(i + 1, len(nums)):
+                for k in range(j + 1, len(nums)):
+                    val = target - (nums[i] + nums[j] + nums[k])
+                    if val in hashmap:
+                        # 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
+        
+```
 
 Go：