关于python递归的困惑

Question

我正在编写一个递归函数来解决一个问题，在这个问题上，您实际上是在尝试查找所有组合以达到给定数量的目标总和，并且每个数字都小于特定限制。

我编写的代码如下：

results = []

def findSchedulesHelper(workHours_left, dayHoursLimit, days_left, res):
    if workHours_left < 0:
        return
    elif workHours_left > 0 and days_left == 0:
        return
    elif workHours_left == 0 and days_left != 0:
        for i in range(days_left):
            res.append(0)
        results.append(res)
        return
    elif days_left == 0 and workHours_left == 0:
        results.append(res)
        return
    else:
        for i in range(dayHoursLimit + 1):
            res.append(i)
            findSchedulesHelper(workHours_left - i, dayHoursLimit, days_left - 1, res)
            res.pop()


def findSchedules(workHours, dayHours, pattern):
    # Write your code here
    num_question_mark = 0
    total_hours_so_far = 0
    for char in pattern:
        if char == "?":
            num_question_mark += 1
        else:
            total_hours_so_far += int(char)

    hours_left_to_fill = workHours - total_hours_so_far

    for i in range(dayHours + 1):
        findSchedulesHelper(hours_left_to_fill - i, dayHours, num_question_mark - 1, [i])
    print results

findSchedules(3,2,"??2??00")

但这给出了错误的答案。

如果我更改一行并且做到这一点：

results = []

def findSchedulesHelper(workHours_left, dayHoursLimit, days_left, res):
    if workHours_left < 0:
        return
    elif workHours_left > 0 and days_left == 0:
        return
    elif workHours_left == 0 and days_left != 0:
        for i in range(days_left):
            res.append(0)
        results.append(res)
        return
    elif days_left == 0 and workHours_left == 0:
        results.append(res)
        return
    else:
        for i in range(dayHoursLimit + 1):
            findSchedulesHelper(workHours_left - i, dayHoursLimit, days_left - 1, res + [i])


def findSchedules(workHours, dayHours, pattern):
    # Write your code here
    num_question_mark = 0
    total_hours_so_far = 0
    for char in pattern:
        if char == "?":
            num_question_mark += 1
        else:
            total_hours_so_far += int(char)

    hours_left_to_fill = workHours - total_hours_so_far

    for i in range(dayHours + 1):
        findSchedulesHelper(hours_left_to_fill - i, dayHours, num_question_mark - 1, [i])
    print results

findSchedules(3,2,"??2??00")

然后一切都正确。我真的很困惑为什么会这样。我尝试进行一些调试，发现append和pop未能按我预期的方式工作。如果任何人都可以解释我执行此功能的两种方式之间的确切区别，那将是很好的。