Python 循环列表删除元素的注意事项

错误示范：

 class Solution:
     def removeElement(self, nums, val: int) -> int:
         for i, num in enumerate(nums):
             print('i=', i, ', num=', num, ', nums=', nums)
             if num == val:
                 nums.remove(val)
         return len(nums)

 s = Solution()
 s.removeElement([2, 0,1,2,2,3,0,4,2], 2)
 # i= 0 , num= 2 , nums= [2, 0, 1, 2, 2, 3, 0, 4, 2]
 # i= 1 , num= 1 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 2 , num= 2 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 3 , num= 3 , nums= [0, 1, 2, 3, 0, 4, 2]
 # i= 4 , num= 0 , nums= [0, 1, 2, 3, 0, 4, 2]
 # i= 5 , num= 4 , nums= [0, 1, 2, 3, 0, 4, 2]
 # i= 6 , num= 2 , nums= [0, 1, 2, 3, 0, 4, 2]

解决方式：

① 使用尾递归方式

 class Solution:
     def removeElement(self, nums, val: int) -> int:
         for i, num in enumerate(nums[::-1]):
             print('i=', i, ', num=', num, ', nums=', nums)
             if num == val:
                 nums.remove(val)
         return len(nums)

 s = Solution()
 s.removeElement([2, 0,1,2,2,3,0,4,2], 2)
 # i= 0 , num= 2 , nums= [2, 0, 1, 2, 2, 3, 0, 4, 2]
 # i= 1 , num= 4 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 2 , num= 0 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 3 , num= 3 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 4 , num= 2 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 5 , num= 2 , nums= [0, 1, 2, 3, 0, 4, 2]
 # i= 6 , num= 1 , nums= [0, 1, 3, 0, 4, 2]
 # i= 7 , num= 0 , nums= [0, 1, 3, 0, 4, 2]
 # i= 8 , num= 2 , nums= [0, 1, 3, 0, 4, 2]

② 使用 while 循环的方式

 class Solution:
     def removeElement(self, nums, val: int) -> int:
         i = 0
         while i < len(nums):
             print('i=', i, ', num=', nums[i], ', nums=', nums)
             if nums[i] == val:
                 nums.pop(i)
             else:
                 i += 1
         return len(nums)

 s = Solution()
 s.removeElement([2, 0,1,2,2,3,0,4,2], 2)
 # i= 0 , num= 2 , nums= [2, 0, 1, 2, 2, 3, 0, 4, 2]
 # i= 0 , num= 0 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 1 , num= 1 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 2 , num= 2 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 2 , num= 2 , nums= [0, 1, 2, 3, 0, 4, 2]
 # i= 2 , num= 3 , nums= [0, 1, 3, 0, 4, 2]
 # i= 3 , num= 0 , nums= [0, 1, 3, 0, 4, 2]
 # i= 4 , num= 4 , nums= [0, 1, 3, 0, 4, 2]
 # i= 5 , num= 2 , nums= [0, 1, 3, 0, 4, 2]

③ 对整个序列使用切片来创建一个临时副本

 class Solution:
     def removeElement(self, nums, val: int) -> int:
         for i, num in enumerate(nums[:]):
             print('i=', i, ', num=', num, ', nums=', nums)
             if num == val:
                 nums.remove(val)
         return len(nums)

 s = Solution()
 s.removeElement([2, 0,1,2,2,3,0,4,2], 2)
 # i= 0 , num= 2 , nums= [2, 0, 1, 2, 2, 3, 0, 4, 2]
 # i= 1 , num= 0 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 2 , num= 1 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 3 , num= 2 , nums= [0, 1, 2, 2, 3, 0, 4, 2]
 # i= 4 , num= 2 , nums= [0, 1, 2, 3, 0, 4, 2]
 # i= 5 , num= 3 , nums= [0, 1, 3, 0, 4, 2]
 # i= 6 , num= 0 , nums= [0, 1, 3, 0, 4, 2]
 # i= 7 , num= 4 , nums= [0, 1, 3, 0, 4, 2]
 # i= 8 , num= 2 , nums= [0, 1, 3, 0, 4, 2]