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python - 如何收集递归回溯的结果？

问题描述

我正在自学递归回溯。对于骰子求和问题，我不知道如何优雅地收集结果。

作为参考，这是我的代码，它只打印任何符合标准的骰子。理想情况下，我想改变它，而不是打印输出，我可以建立一个列表，列出那些选择的骰子并返回它。

下面是不符合我要求的代码

def dice_sum(num_dice: int, target_sum: int) -> None:
    dice_sum_helper(num_dice, target_sum, [])


def dice_sum_helper(num_dice: int, target_sum: int, chosen: List[int]) -> None:
    if num_dice == 0 and sum(chosen) == target_sum:
        print(chosen)
    elif num_dice == 0:
        pass
    else:
        for i in range(1, 7):
            chosen.append(i)
            dice_sum_helper(num_dice - 1, target_sum, chosen)
            chosen.pop()

相反，我希望它做这样的事情

from typing import List
DiceResult = List[List[int]]


def dice_sum(num_dice: int, target_sum: int) -> DiceResult:
    return dice_sum_helper(num_dice, target_sum, [])


def dice_sum_helper(num_dice: int, target_sum: int, chosen: List[int]) -> DiceResult:
    if num_dice == 0 and sum(chosen) == target_sum:
        # Return the value that meets the constraints
        return chosen
    elif num_dice == 0:
        pass
    else:
        for i in range(1, 7):
            chosen.append(i)
            # Return the result of my recursive call and build the list of lists?
            result = dice_sum_helper(num_dice - 1, target_sum, chosen)
            return result.append(result)
            # End of that logic
            chosen.pop()

我正在寻找要使用的理论或模式，而不是确切的代码。如果不使用外部列表，我无法完全获取代码来收集和附加每个结果。

标签： pythonrecursionbacktrackingrecursive-backtracking

解决方案

您可以利用yield并yield from从您的函数返回结果：

from typing import List

def dice_sum(num_dice: int, target_sum: int) -> None:
    yield from dice_sum_helper(num_dice, target_sum, [])


def dice_sum_helper(num_dice: int, target_sum: int, chosen: List[int]) -> None:
    if num_dice == 0 and sum(chosen) == target_sum:
        yield chosen[:]
    elif num_dice == 0:
        pass
    else:
        for i in range(1, 7):
            chosen.append(i)
            yield from dice_sum_helper(num_dice - 1, target_sum, chosen)
            chosen.pop()

# you can store the results e.g. to list: 
# results = list(dice_sum(3, 12))

for dices in dice_sum(3, 12):
    for d in dices:
        print('{: ^4}'.format(d), end='|')
    print()

印刷：

 1  | 5  | 6  |
 1  | 6  | 5  |
 2  | 4  | 6  |
 2  | 5  | 5  |
 2  | 6  | 4  |
 3  | 3  | 6  |
 3  | 4  | 5  |
 3  | 5  | 4  |
 3  | 6  | 3  |
 4  | 2  | 6  |
 4  | 3  | 5  |
 4  | 4  | 4  |
 4  | 5  | 3  |
 4  | 6  | 2  |
 5  | 1  | 6  |
 5  | 2  | 5  |
 5  | 3  | 4  |
 5  | 4  | 3  |
 5  | 5  | 2  |
 5  | 6  | 1  |
 6  | 1  | 5  |
 6  | 2  | 4  |
 6  | 3  | 3  |
 6  | 4  | 2  |
 6  | 5  | 1  |

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