Make Change Problem，找零钱问题

假设你去超时买东西，不用微信，而是用传统的纸币，收银员要给你找零钱，只有5元，2元和1元的纸币，要找给你7元，每种面值的零钱无限量供应，一共有多少种不同的零钱组合方式找给你。注意：每种面值的零钱无限量供应！

7 = 1*7
7 = 2*3 + 1
7 = 2*2 + 1*3
7 = 2 + 1*5
7 = 5 + 2
7 = 5 + 1*2

递归

def make_change(amount: int, coins: tuple[int]) -> int:
    if amount == 0:
        return 1
    if amount < 0 or not coins:
        return 0
    i = make_change(amount-coins[0], coins)
    j = make_change(amount, coins[1:])
    return i+j


assert make_change(0,(5,2,1)) == 1
assert make_change(1,(5,2,1)) == 1
assert make_change(2,(5,2,1)) == 2
assert make_change(3,(5,2,1)) == 2
assert make_change(4,(5,2,1)) == 3
assert make_change(5,(5,2,1)) == 4
assert make_change(6,(5,2,1)) == 5
assert make_change(7,(5,2,1)) == 6

最后把这两个子问题的结果相加。这两个子问题，绝对不会重合！

现在我们将此问题更进一步，不是计算组合数量，而是把每一种具体的组合找出来，实现如下：

def make_change_d(amount: int, coins: tuple[int]) -> list[tuple[int,...]]:
    if amount == 0:
        return [()]
    if amount < 0 or not coins:
        return []
    i = [z+(coins[0],) for z in make_change_d(amount-coins[0],coins)]
    j = make_change_d(amount, coins[1:])
    return i+j


assert len(make_change_d(0,(5,2,1))) == 1
assert len(make_change_d(1,(5,2,1))) == 1
assert len(make_change_d(2,(5,2,1))) == 2
assert len(make_change_d(3,(5,2,1))) == 2
assert len(make_change_d(4,(5,2,1))) == 3
assert len(make_change_d(5,(5,2,1))) == 4
assert len(make_change_d(6,(5,2,1))) == 5
assert len(make_change_d(7,(5,2,1))) == 6


from pprint import pprint
print('7:')
pprint(make_change_d(7,(5,2,1)))
print('10:')
pprint(make_change_d(10,(5,2,1)))

7:
[(2, 5),
 (1, 1, 5),
 (1, 2, 2, 2),
 (1, 1, 1, 2, 2),
 (1, 1, 1, 1, 1, 2),
 (1, 1, 1, 1, 1, 1, 1)]
10:
[(5, 5),
 (1, 2, 2, 5),
 (1, 1, 1, 2, 5),
 (1, 1, 1, 1, 1, 5),
 (2, 2, 2, 2, 2),
 (1, 1, 2, 2, 2, 2),
 (1, 1, 1, 1, 2, 2, 2),
 (1, 1, 1, 1, 1, 1, 2, 2),
 (1, 1, 1, 1, 1, 1, 1, 1, 2),
 (1, 1, 1, 1, 1, 1, 1, 1, 1, 1)]

DP递归

上述递归算法非常慢，Big-Oh为\(O(2^N)\)，如果采用DP思路，将计算过程中的子问题的结果记录下来，在后续计算中直接使用，同样是递归，速度变得非常快！

result = {}
def make_change_dp_recursive(amount: int, coins: tuple[int]) -> int:
    global result
    if amount == 0:
        return 1
    if amount < 0 or not coins:
        return 0

    k = (amount-coins[0], coins)
    try:
        i = result[k]
    except KeyError:
        i = result[k] = make_change_dp_recursive(*k)

    k = (amount, coins[1:])
    try:
        j = result[k]
    except KeyError:
        j = result[k] = make_change_dp_recursive(*k)

    return i+j

直接取曾经计算过的子问题的结果，不再重复计算。如果计算(210,(5,2,1))，改进后的算法要快超过100倍！

run (210, (5, 2, 1)) with number = 200
make_change return: 2290
make_change_dp_recursive return: 2290
make_change average cpu time: 0.0429869594
make_change_dp_recursive average cpu time: 0.00041894424499999694
boost: 102.60782887708676

改进后的算法，Big-Oh是\(O(M\times N)\)，M是amount的值大小，N是硬币种类的数量，因为每个子问题，只需要计算一次，后面再遇到，就是直接取值！\(M\times N\)就是所有子问题的数量。

下面是在make_change_dp_recursive基础上改出来的计算出所有组合的代码：

res = {}
def make_change_dp_recursive_d(amount: int, coins: tuple[int]) -> list[tuple[int,...]]:
    global res
    if amount == 0:
        return [()]
    if amount < 0 or not coins:
        return []

    k = (amount, coins)
    try:
        i = res[k]
    except KeyError:
        i = res[k] = [z+(coins[0],)
                      for z in make_change_dp_recursive_d(amount-coins[0],coins)]

    k = (amount, coins[1:])
    try:
        j = res[k]
    except KeyError:
        j = res[k] = make_change_dp_recursive_d(*k)

    return i+j

DP循环

很多DP问题，都可以使用自底向上的循环的方式，一层层计算子问题，最后得到问题的解。找零问题也可以使用这个思路，代码如下：

def make_change_loop(amount: int, coins: tuple[int]) -> int:
    w = [0] * (amount+1)  # faster than listcomp
    w[0] = 1
    for c in coins:
        for i in range(c, amount+1):
            w[i] += w[i-c]
    return w[-1]


assert make_change_loop(0,(5,2,1)) == make_change(0,(5,2,1))
assert make_change_loop(1,(5,2,1)) == make_change(1,(5,2,1))
assert make_change_loop(2,(5,2,1)) == make_change(2,(5,2,1))
assert make_change_loop(3,(5,2,1)) == make_change(3,(5,2,1))
assert make_change_loop(4,(5,2,1)) == make_change(4,(5,2,1))
assert make_change_loop(5,(5,2,1)) == make_change(5,(5,2,1))
assert make_change_loop(6,(5,2,1)) == make_change(6,(5,2,1))
assert make_change_loop(7,(5,2,1)) == make_change(7,(5,2,1))

run (210, (5, 2, 1)) with number = 200
make_change return: 2290
make_change_loop return: 2290
make_change average cpu time: 0.04304871445
make_change_loop average cpu time: 4.425439500000294e-05
boost: 972.755687881331

def make_change_loop_d(amount: int, coins: tuple[int]) -> list[tuple[int,...]]:
    w = [[] for i in range(amount+1)]  # cannot use [[]]*N
    w[0].append(())
    for c in coins:
        for i in range(c, amount+1):
            w[i] += [z+(c,) for z in w[i-c]]
    return w[-1]

总结

>>> import sys
>>> sys.setrecursionlimit(100)
>>> sys.getrecursionlimit()
100
>>> make_change(96,(5,2,1))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/xinlin/test/tt.py", line 7, in make_change
    j = make_change(amount, coins[1:])
  File "/home/xinlin/test/tt.py", line 7, in make_change
    j = make_change(amount, coins[1:])
  File "/home/xinlin/test/tt.py", line 6, in make_change
    i = make_change(amount-coins[0], coins)
  File "/home/xinlin/test/tt.py", line 6, in make_change
    i = make_change(amount-coins[0], coins)
  File "/home/xinlin/test/tt.py", line 6, in make_change
    i = make_change(amount-coins[0], coins)
  [Previous line repeated 93 more times]
  File "/home/xinlin/test/tt.py", line 2, in make_change
    if amount == 0:
RecursionError: maximum recursion depth exceeded in comparison

DP递归方案稍微好一点，但用处还是不大，虽然由于直接取子问题的值可以缓解递归深度的问题，但是并不能从更本上解决此问题，递归深度的问题，与输入密切相关：

>>> make_change_dp_recursive(96,(5,2,1))
500
>>> make_change_dp_recursive(96,(1,2,5))
...
RecursionError: maximum recursion depth exceeded in comparison

Make Change Problem，找零钱问题

递归

DP递归

DP循环

总结

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