# Project Euler 37

• 問題

Problem 37：Truncatable primes
The number 3797 has an interesting property. Being prime itself, it is possible to continuously remove digits from left to right, and remain prime at each stage: 3797, 797, 97, and 7. Similarly we can work from right to left: 3797, 379, 37, and 3.

Find the sum of the only eleven primes that are both truncatable from left to right and right to left.

NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.

• 解答例
```def isPrime(num):
if num <= 1:
return False
elif num == 2 or num == 3:
return True
if num % 2 == 0 or num % 3 == 0:
return False
if not(num % 6 == 1) and not(num % 6 == 5):
return False
i = 5
while i * i <= num:
if num % i == 0 or num % (i + 2) == 0:
return False
i += 6
return True

ans = []
num = 21
while True:
if not isPrime(num):
num += 1
continue
# check left side
tempLeft = num
isLeftPrime = True
temp = list(str(tempLeft))
l = len(temp)
for i in range(l - 1):
temp.pop(0)
tempNum = int("".join(temp))
if not isPrime(tempNum):
isLeftPrime = False
break
if not isLeftPrime:
num += 1
continue
# check right side
tempRight = num
isRightPrime = True
temp = list(str(tempRight))
l = len(temp)
for i in range(l - 1):
temp.pop()
tempNum = int("".join(temp))
if not isPrime(tempNum):
isRightPrime = False
break
if not isRightPrime:
num += 1
continue

ans.append(num)
num += 1

if len(ans) == 11:
break

print(sum(ans))
```