记一次浅拷贝的错误

1 import copy 2 class Solution(object): 3 def subsets(self, nums): 4 """ 5 :type nums: List[int] 6 :rtype: List[List[int]] 7 """ 8 result = [[]] 9 self.generate(0,[],result,nums) 10 return result 11 def generate(self,k,item,result,nums): 12 if k > len(nums)-1: 13 return 14 item.append(nums[k]) 15 result.append(copy.deepcopy(item))#result.append(item) 16 self.generate(k+1,item,result,nums) 17 item.pop() 18 self.generate(k+1,item,result,nums)

leetcode刷题时遇到了78题Subset，写了上面的代码，在第15行最开始用了注释里的result.append(item)，把item的引用（地址）传给了result，结果每次递归result中除了原有的‘[]’元素，其他都同步在变。

下面是nums=[1,2,3]时，使用result.append(item)语句的过程

item: [1] result: [[], [1]] item: [1, 2] result: [[], [1, 2], [1, 2]] item: [1, 2, 3] result: [[], [1, 2, 3], [1, 2, 3], [1, 2, 3]] item: [1, 3] result: [[], [1, 3], [1, 3], [1, 3], [1, 3]] item: [2] result: [[], [2], [2], [2], [2], [2]] item: [2, 3] result: [[], [2, 3], [2, 3], [2, 3], [2, 3], [2, 3], [2, 3]] item: [3] result: [[], [3], [3], [3], [3], [3], [3], [3]]

下面是深拷贝时的过程

item: [1] result: [[], [1]] item: [1, 2] result: [[], [1], [1, 2]] item: [1, 2, 3] result: [[], [1], [1, 2], [1, 2, 3]] item: [1, 3] result: [[], [1], [1, 2], [1, 2, 3], [1, 3]] item: [2] result: [[], [1], [1, 2], [1, 2, 3], [1, 3], [2]] item: [2, 3] result: [[], [1], [1, 2], [1, 2, 3], [1, 3], [2], [2, 3]] item: [3] result: [[], [1], [1, 2], [1, 2, 3], [1, 3], [2], [2, 3], [3]]

 

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