PAT（甲级）2020年春季考试7-1 Prime Day (20分)

The above picture is from Sina Weibo, showing May 23rd, 2019 as a very cool "Prime Day". That is, not only that the corresponding number of the date 20190523 is a prime, but all its sub-strings ended at the last digit 3 are prime numbers.

Now your job is to tell if a given date is a Prime Day.

Input Specification:

Each input file contains one test case. For each case, a date between January 1st, 0001 and December 31st, 9999 is given, in the format yyyymmdd.

Output Specification:

For each given date, output in the decreasing order of the length of the substrings, each occupies a line. In each line, print the string first, followed by a space, then Yes if it is a prime number, or No if not. If this date is a Prime Day, print in the last line All Prime!.

Sample Input 1:

20190523

Sample Output 1:

20190523 Yes 0190523 Yes 190523 Yes 90523 Yes 0523 Yes 523 Yes 23 Yes 3 Yes All Prime!

Sample Input 2:

20191231

Sample Output 2:

20191231 Yes 0191231 Yes 191231 Yes 91231 No 1231 Yes 231 No 31 Yes 1 No

思路

很简单的字符串处理题，用sunstr获取子字符串。注意素数需要考虑1不是素数的情况，不然会有两个点通过不了。

代码 

#include<stdio.h> #include<iostream> #include<math.h> #include<string> using namespace std; bool flag=true; bool isprime(int n){ if(n==1) return false; for(int i=2;i<sqrt(n);i++){ if(n%i==0){ flag=false; return false; } } return true; } int main(){ string s,temp; int num; cin>>s; for(int i=0;i<s.length();i++){ temp=s.substr(i,s.length()); num=stoi(temp); printf("%s %s\n",temp.c_str(),isprime(num)?"Yes":"No"); } if(flag) printf("All Prime!"); return 0; }