867. Prime Palindrome

Find the smallest prime palindrome greater than or equal to N.

Recall that a number is prime if it's only divisors are 1 and itself, and it is greater than 1. 

For example, 2,3,5,7,11 and 13 are primes.

Recall that a number is a palindrome if it reads the same from left to right as it does from right to left. 

For example, 12321 is a palindrome.

 

Example 1:

Input: 6
Output: 7

Example 2:

Input: 8
Output: 11

Example 3:

Input: 13
Output: 101

 

Note:

• 1 <= N <= 10^8
• The answer is guaranteed to exist and be less than 2 * 10^8.

发现有个规律ABCCBA，ABCDDCBA这种偶数形式的一定不是质数（11除外），一定是11的倍数，所以只要找奇数位长度的数了

然后以为穷举+判断是不是prime即可，搜索的GeeksForGeeks的答案都是求出所有质数，然后循环，好无语，

自己真应该先试一下自己的穷举+判断是不是prime的想法的，乱七八糟的搜类似的题还真是浪费很多时间

class Solution:
    def primePalindrome(self, n):
        """
        :type N: int
        :rtype: int
        """
        def isPrime(t):
            if t==1: return False
            i = 2
            while i*i<=t:
                if t%i==0: return False
                i+=1
            return True
        
        if n<110:
            for i in range(n, 1000000):
                if str(i)==str(i)[::-1] and isPrime(i): return i
             
        s=str(n)
        if len(s)%2==0:
            p=len(s)//2
            t=int('1'+'0'*(p))
        else:
            p=len(s)//2
            t=int(s[:p+1])
            if int(s[:p+1][::-1])<int(s[p:]): t+=1
        
        while 1:  
            s = str(t)
            res = int(s+s[:-1][::-1])
            if isPrime(int(res)):
                return res
            else:
                t+=1

s=Solution()
#print(s.primePalindrome(1))
#print(s.primePalindrome(2))
#print(s.primePalindrome(6))
#print(s.primePalindrome(8))
#print(s.primePalindrome(13))
#print(s.primePalindrome(758))
#print(s.primePalindrome(9015110))
print(s.primePalindrome(85709140))