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Kurt reaches nirvana when he finds the product of all the digits of some positive integer. Greater value of the product makes the nirvana deeper.

Help Kurt find the maximum possible product of digits among all integers from 1 to n.

The only input line contains the integer nn (

Print the maximum product of digits among all integers from 1 to n.

390

216

7

7

1000000000

387420489

In the first example the maximum product is achieved for

In the second example the maximum product is achieved for 77 (the product of digits is 7).

In the third example the maximum product is achieved for 99999999 (the product of digits is 9⁹=38742048999).

題意：給出n，找出不大於n的一個數，試其乘積最大。

思路：對於一個位置上的數，①可以保持不變，②可以使其變成9，前置位-1.

這樣我們可以遞歸枚舉。

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 
 4 int n;
 5 
 6 int cal(int n)
 7 {
 8     if(n == 0)return 1;
 9     else if(n < 10)return n;
10     else
11     {
12         return max(cal(n/10)*(n%10),cal(n/10-1)*9);
13     }
14 }
15 
16 int main()
17 {
18     scanf("%d",&n);
19     printf("%d\n",cal(n));
20 }

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B. Nirvana Codeforces Round #549 (Div. 2) （遞歸dfs）