題目一：

John and Mary want to travel between a few towns A, B, C ... Mary has on a sheet of paper a list of distances between these towns. ls = [50, 55, 57, 58, 60]. John is tired of driving and he says to Mary that he doesn't want to drive more than t = 174 miles and he will visit only 3

 towns.

Which distances, hence which towns, they will choose so that the sum of the distances is the biggest possible

• to please Mary and John- ?

Example:

With list ls and 3 towns to visit they can make a choice between:[50,55,57],[50,55,58],[50,55,60],[50,57,58],[50,57,60],[50,58,60],[55,57,58],[55,57,60],[55,58,60],[57,58,60]

.

The sums of distances are then: 162, 163, 165, 165, 167, 168, 170, 172, 173, 175.

The biggest possible sum taking a limit of 174 into account is then 173 and the distances of the 3 corresponding towns is [55, 58, 60].

The function chooseBestSum (or choose_best_sum

 or ... depending on the language) will take as parameters t (maximum sum of distances, integer >= 0), k (number of towns to visit, k >= 1) and ls (list of distances, all distances are positive or null integers and this list has at least one element). The function returns the "best" sum ie the biggest possible sum of k distances less than or equal to the given limit t, if that sum exists, or otherwise nil, null, None, Nothing, depending on the language. With C++, C, Rust, Swift, Go, Kotlin return -1.

Examples:

ts = [50, 55, 56, 57, 58] choose_best_sum(163, 3, ts) -> 163

xs = [50] choose_best_sum(163, 3, xs) -> nil (or null or ... or -1 (C++, C, Rust, Swift, Go)

ys = [91, 74, 73, 85, 73, 81, 87] choose_best_sum(230, 3, ys) -> 228

題意：
題目給了choose_best_sum(t, k, ls)，ls是一堆距離，題意就是要我們找出ls中k個數相加的最大值且不超過t的大小；
這道題真的麻煩，但是我在網上查到了一個python的函式叫做組合函式，combinations；比如ls=[1，2，3，4],k=2,用了全排列函式就可以找出所有的2個數組合的情況。比如[1,2]、[1,3]、[1,4]、[2,3]……

思路是：
算出所有的距離之和找出最大且不超過ls的大小的距離。

程式碼如下：
from itertools import combinations
def choose_best_sum(t, k, ls):
    sum2=0
    for dists in combinations(ls,k):
        sum1=sum(dists)
        if sum1>t:
            continue
        sum2=max(sum2,sum1)
    if sum2==0:
        return None
    return sum2

看別人的精簡程式碼：

from itertools import combinations
 
 
def choose_best_sum(t, k, ls):
    return max((s for s in (sum(dists) for dists in combinations(ls, k)) if s <= t), default=None)
  

思路是一樣的，但是這個程式碼是極其精簡的。