优化 0027.移除元素.md Python3解法

problems/0027.移除元素.md

@@ -144,15 +144,28 @@ class Solution {
 
 Python：
 
-```python
+```python3
 class Solution:
-    def removeElement(self, nums: List[int], val: int) -> int:
-        i,n = 0,len(nums)
-        for j in range(n):
-            if nums[j] != val:
-                nums[i] = nums[j]
-                i += 1
-        return i
+    """双指针法
+    时间复杂度：O(n)
+    空间复杂度：O(1)
+    """
+
+    @classmethod
+    def removeElement(cls, nums: List[int], val: int) -> int:
+        fast = slow = 0
+
+        while fast < len(nums):
+
+            if nums[fast] != val:
+                nums[slow] = nums[fast]
+                slow += 1
+
+            # 当 fast 指针遇到要删除的元素时停止赋值
+            # slow 指针停止移动, fast 指针继续前进
+            fast += 1
+
+        return slow
 ```