Update 0027.移除元素.md

the former code has not consider if nums is None or len(nums)==0

Author: janeyziqinglin

problems/0027.移除元素.md

@@ -173,28 +173,24 @@ class Solution {
 
 Python：
 
-```python
+```python3
 class Solution:
-    """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
+    def removeElement(self, nums: List[int], val: int) -> int:
+        if nums is None or len(nums)==0: 
+            return 0 
+        l=0
+        r=len(nums)-1
+        while l<r: 
+            while(l<r and nums[l]!=val):
+                l+=1
+            while(l<r and nums[r]==val):
+                r-=1
+            nums[l], nums[r]=nums[r], nums[l]
+        print(nums)
+        if nums[l]==val: 
+            return l 
+        else: 
+            return l+1
 ```