Why do we need to copy array element inside backtracking

Question

This is a classic backtracking problem , i dont know why we need to copy arr element inside recursive function in order for it to work correctly. If i dont copy. It will return blank array

def subsets(self, nums: List[int]) -> List[List[int]]:
        res = list()
        temp = list()
        def dfs(nums,i):
            if i==len(nums):
                res.append(temp.copy())
                return
            temp.append(nums[i])
            dfs(nums,i+1)
            temp.pop()
            dfs(nums,i+1)
        dfs(nums,0)
        return res

Does this answer your question? [Python: List of Objects changes when the object that was input in the append() function changes](https://stackoverflow.com/questions/24345712/python-list-of-objects-changes-when-the-object-that-was-input-in-the-append-f) — SomeDude, Jul 07 '22 at 21:10

score 2 · Answer 1 · answered Jul 07 '22 at 21:10

the reason is that, when you append nums to results, nums can still be changed even if it's inside results. Therefore the elements inside results will be changed everytime you change the original nums (in fact in the result all the values are identical). If you create a copy for each element you put in results, instead, the elements will all have different values.

Why do we need to copy array element inside backtracking

1 Answers1