Update 0018.四数之和.md

problems/0018.四数之和.md

@@ -207,35 +207,40 @@ class Solution {
 ```
 
 Python：
+(版本一) 双指针
 ```python
-# 双指针法
-class Solution:
-    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
-        
+ def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
+        result = []
         nums.sort()
-        n = len(nums)
-        res = []
-        for i in range(n):   
-            if i > 0 and nums[i] == nums[i - 1]: continue  # 对nums[i]去重
-            for k in range(i+1, n):
-                if k > i + 1 and nums[k] == nums[k-1]: continue  # 对nums[k]去重
-                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
+        for k in range(len(nums)):
+            if nums[k] > target and nums[k] >= 0:
+                break
+            if k > 0 and nums[k] == nums[k-1]:
+                continue
+            for i in range(k+1, len(nums)):
+                if nums[k] + nums[i] > target and nums[k] + nums[i] >= 0:
+                    break
+                if i > k+1 and nums[i] == nums[i-1]:
+                    continue
+                left, right = i+1, len(nums)-1
+                while right > left:
+                    if nums[k] + nums[i] + nums[left] + nums[right] > target:
+                        right -= 1
+                    elif nums[k] + nums[i] + nums[left] + nums[right] < target:
+                        left += 1
                     else:
-                        res.append([nums[i], nums[k], nums[p], nums[q]])
-			# 对nums[p]和nums[q]去重
-                        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
+                        result.append([nums[k], nums[i], nums[left], nums[right]])
+                        while right > left and nums[right] == nums[right-1]:
+                            right -= 1
+                        while right > left and nums[left] == nums[left+1]:
+                            left += 1
+                        right -= 1
+                        left += 1
+        return result
 ```
+(版本二) 使用字典
+
 ```python
-# 哈希表法
 class Solution(object):
     def fourSum(self, nums, target):
         """