PAT 2020年春季 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

实现思路：

一道水题，就是截取字符串判断对应数字是否都是素数。

AC代码：

#include <iostream>
#include <cmath>
#include <cstring>
using namespace std;

bool isPrime(int x) {
	if(x<=1) return false;
	int sqr=sqrt(1.0*x);
	for(int i=2; i<=sqr; i++) {
		if(x%i==0) return false;
	}
	return true;
}

int main() {
	string str;
	cin>>str;
	bool res=true;
	int Num=str.length();
	for(int i=0; i<Num; i++) {
		cout<<str<<" ";
		if(isPrime(stoi(str))) printf("Yes");
		else {
			res=false;
			printf("No");
		}
		cout<<endl;
		str.erase(str.begin());
	}
	if(res) printf("All Prime!");
	return 0;
}