组合python非常大的列表

Question

I need a list of all possible combinations of k out of n numbers for very large n. I run into the problem of my list size exceeding 500000 quickly, which python does not seem to handle very well. However, I would like to exceed that by hundreds of millions. How do I handle that? Thanks a lot!

Answer 1

DP won't work for very large number because storing such large size list would result in MemoryError . Idea is to do calculate things on fly and record the required values.

def comb(n, k):
        deno = 1
        num = 1
        result = 1
        #set the value of deno and num bounds
        if k > n - k:
                num_val = k + 1
                den_val = n - k
        else:
                num_val = n - k + 1
                den_val = k
        i = n
        #calculate n * (n - k) *....* (n-num_value))
        while i != num_val - 1:
                if i == 0:
                        result *= 1
                else:
                        result *= i
                if i == num_val:
                        num = result
                i -=1
        i = 0
        result = 1
        #calculate den_val! (factorial of denominator)
        while i != den_val + 1:
                if i == 0:
                        result *= 1
                else:
                        result *= i
                if i == den_val:
                        deno = result
                i += 1
        ans = num/deno
        return ans


if __name__ == "__main__":
        n = input("Enter n\n")
        k = input("Enter k\n")
        print(comb(n, k))

Complexity - O(max(k, n - k))