Something weird happend to my code. (py)

  • 1

    My code:

    class Solution:
      def combine(self, n, k):
        return self.combine2(n, k)
      def combine2(self, n, k, offset=1, collects=[], prev=[]):
        if k == 0:
            return collects.append(prev)
        for i in range(offset, n-k+2):
            me = list(prev)
            self.combine2(n, k-1, i+1, collects, me)
        return collects

    I think it is correct anwser.

    BUT I always got this result:

    Submission Result: Wrong Answer
    Input:	2, 1
    Output:	[[1],[1],[2]]
    Expected:	[[1],[2]]

    I have no idea what just happened.....

  • 2

    Hi e39a562r:

    When a function takes an argument with default parameter, it is immediately re-evaluated when the function returns. So, when you call ‘combine2’ multiple times, the value of 'collects' is always changing, even it is set default. In your case, the first ‘[1]’ in your answer is the result of last test case (input may be (1,1) I guess).

    So when calling combine2, use self.combine2(n, k, 1, [], []) instead, and change the definition of combine2 to def combine2(self, n, k, offset, collects, prev): will work.

    Your may refer to the Python Manual for more details.

  • 0

    I got it. Thanks!
    It seems dict and list shouldn't assign to default parameter......

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