python类属性更新问题

Question

我编写了一个小程序来打印出数组的所有排列 - 假设输入是 [1,2,3]。我正在使用回溯来生成解决方案。

我的问题是我可以看到结果已更新为所需值(基于打印类属性 result 的日志)但未返回。实际上返回的答案是 [[],[],[],[],[],[]] – 这是我不明白的。

class Solution:
    def __init__(self):
        self.result = []
    def permute(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        c = self.permuteHelper(nums, [])
        print("finally, result:", self.result)
        return self.result 

    def permuteHelper(self, nums, chosen):
        print("permuteHelper: nums", nums, "chosen:", chosen)
        if len(nums) == 0:
            print("returning:", chosen)
            self.result.append(chosen)
            print("result:", self.result)
            return self.result 
        index = 0 
        for n in nums:
            nums.pop(index)
            chosen.append(n)

            c = self.permuteHelper(nums, chosen)

            nums.insert(index, n)
            chosen.remove(n)
            index+=1

这是我得到的输出:

permuteHelper: nums [1, 2, 3] chosen: []
permuteHelper: nums [2, 3] chosen: [1]
permuteHelper: nums [3] chosen: [1, 2]
permuteHelper: nums [] chosen: [1, 2, 3]
returning: [1, 2, 3]
result: [[1, 2, 3]]
permuteHelper: nums [2] chosen: [1, 3]
permuteHelper: nums [] chosen: [1, 3, 2]
returning: [1, 3, 2]
result: [[1, 3, 2], [1, 3, 2]]
permuteHelper: nums [1, 3] chosen: [2]
permuteHelper: nums [3] chosen: [2, 1]
permuteHelper: nums [] chosen: [2, 1, 3]
returning: [2, 1, 3]
result: [[2, 1, 3], [2, 1, 3], [2, 1, 3]]
permuteHelper: nums [1] chosen: [2, 3]
permuteHelper: nums [] chosen: [2, 3, 1]
returning: [2, 3, 1]
result: [[2, 3, 1], [2, 3, 1], [2, 3, 1], [2, 3, 1]]
permuteHelper: nums [1, 2] chosen: [3]
permuteHelper: nums [2] chosen: [3, 1]
permuteHelper: nums [] chosen: [3, 1, 2]
returning: [3, 1, 2]
result: [[3, 1, 2], [3, 1, 2], [3, 1, 2], [3, 1, 2], [3, 1, 2]]
permuteHelper: nums [1] chosen: [3, 2]
permuteHelper: nums [] chosen: [3, 2, 1]
returning: [3, 2, 1]
result: [[3, 2, 1], [3, 2, 1], [3, 2, 1], [3, 2, 1], [3, 2, 1], [3, 2, 1]]
finally, result: [[], [], [], [], [], []]

Answer 1

最简单的修复方法(添加复制):

def permuteHelper(self, nums, chosen):
    print("permuteHelper: nums", nums, "chosen:", chosen)
    if len(nums) == 0:
        print("returning:", chosen)
        self.result.append(copy.copy(chosen))  # note!
        print("result:", self.result)
        return self.result
    index = 0
    for n in nums:
        nums.pop(index)
        chosen.append(n)

        c = self.permuteHelper(nums, chosen)
        nums.insert(index, n)
        chosen.remove(n)
        print("see what happen in here:", self.result)  # error happens here
        index += 1

此链接可能有帮助: How do I pass a variable by reference?