Question

我正在尝试解决此问题：

https://leetcode.com/problems/maximum-swap/description/

我的方法基本上是“检查号码中的第一位是否是最大位数。如果不是，则将其与最大位数互换。如果是，则将列表中的第一位放在一边（称为开头），然后对新号码（第一个数字被砍掉的号码）执行相同的操作。”

为此，我编写了一个称为swapper的递归函数，该函数在其他地方调用。我将数字列表输入其中。如果

，它可以正常工作

def swapper(digits, beginning):
    print 'running: digits, beginning: ', digits, beginning
    if len(digits) > 0: #If there is anything left in the number, continue checking if the first digit is the largest)
        if digits.index(max(digits)) == 0:
            beginning.append(digits[0])
            print 'iterating: digits, beginning = ', digits[1:], beginning
            swapper(digits[1:], beginning) #recursively run the function with a smaller digit list
        else:
            digits[digits.index(max(digits))], digits[0] = digits[0], 
max(digits)
            return beginning + digits
    else: #If there is nothing left in the number, return the beginning list
          #which is just all the digits at this point
        print 'returning beginning'
        print 'beginning: ', beginning
        return beginning

#pseudocode here: digits = digits(digits of the number) 
results = swapper(digits, [])
print 'results: ', results

如果它从未在交换器中命中注释为“ else”的东西，它就可以工作，但是，如果确实如此，它似乎什么也不返回。使用示例编号9973，标准输出为：

running: digits, beginning:  [9, 9, 7, 3] []
iterating: digits, beginning =  [9, 7, 3] [9]
running: digits, beginning:  [9, 7, 3] [9]
iterating: digits, beginning =  [7, 3] [9, 9]
running: digits, beginning:  [7, 3] [9, 9]
iterating: digits, beginning =  [3] [9, 9, 7]
running: digits, beginning:  [3] [9, 9, 7]
iterating: digits, beginning =  [] [9, 9, 7, 3]
running: digits, beginning:  [] [9, 9, 7, 3]
returning beginning
beginning:  [9, 9, 7, 3]
results:  None

看起来它正确地将数字分区到开始列表中。然后，它会正确打印“ beginning：”开头。该列表存在，并且像我期望的那样是[9,9,7,3]。然后应该返回它。但是，当我在函数之外打印结果时，它只是None。为什么？

它可以通过另一条return语句正确返回（该语句显示“ return starting + digits”）。

Answer 1

虽然在构建解决方案并更深入地进行递归时函数可以正常工作，但问题是当您到达基本情况后开始尝试退出时。到目前为止，您的代码不会将递归调用的结果传递回上面的调用。

您需要返回递归调用的结果。在您的情况下，将return放在递归调用swapper(digits[1:], beginning)前面即可。

这就是所有的样子：

def swapper(digits, beginning):
    print 'running: digits, beginning: ', digits, beginning
    if len(digits) > 0: #If there is anything left in the number, continue checking if the first digit is the largest)
        if digits.index(max(digits)) == 0:
            beginning.append(digits[0])
            print 'iterating: digits, beginning = ', digits[1:], beginning
            # ADDED: return
            return swapper(digits[1:], beginning) #recursively run the function with a smaller digit list
        else:
            digits[digits.index(max(digits))], digits[0] = digits[0], max(digits)
            return beginning + digits
    else: #If there is nothing left in the number, return the beginning list
          #which is just all the digits at this point
        print 'returning beginning'
        print 'beginning: ', beginning
        return beginning

#pseudocode here: digits = digits(digits of the number) 
results = swapper([9, 9, 7, 3],[])
print('results: ', results)

输出：

running: digits, beginning:  [9, 9, 7, 3] []
iterating: digits, beginning =  [9, 7, 3] [9]
running: digits, beginning:  [9, 7, 3] [9]
iterating: digits, beginning =  [7, 3] [9, 9]
running: digits, beginning:  [7, 3] [9, 9]
iterating: digits, beginning =  [3] [9, 9, 7]
running: digits, beginning:  [3] [9, 9, 7]
iterating: digits, beginning =  [] [9, 9, 7, 3]
running: digits, beginning:  [] [9, 9, 7, 3]
returning beginning
beginning:  [9, 9, 7, 3]
results: [9, 9, 7, 3]

递归函数不返回任何内容，即使它看起来像

1 个答案: