添加0018.四数之和双指针法python3版本

problems/0018.四数之和.md

@@ -167,7 +167,33 @@ class Solution {
 
 Python：
 ```python
+# 双指针法
+class Solution:
+    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
         
+        nums.sort()
+        n = len(nums)
+        res = []
+        for i in range(n):
+            if i > 0 and nums[i] == nums[i - 1]: continue
+            for k in range(i+1, n):
+                if k > i + 1 and nums[k] == nums[k-1]: continue
+                p = k + 1
+                q = n - 1
+
+                while p < q:
+                    if nums[i] + nums[k] + nums[p] + nums[q] > target: q -= 1
+                    elif nums[i] + nums[k] + nums[p] + nums[q] < target: p += 1
+                    else:
+                        res.append([nums[i], nums[k], nums[p], nums[q]])
+                        while p < q and nums[p] == nums[p + 1]: p += 1
+                        while p < q and nums[q] == nums[q - 1]: q -= 1
+                        p += 1
+                        q -= 1
+        return res
+```
+```python
+# 哈希表法
 class Solution(object):
     def fourSum(self, nums, target):
         """