Project Euler #4 - Largest Palindrome Project - Python

Question

I solved the HackerRank version of the Largest palindrome problem (similar to Project Euler 4) in Python:

Find the largest palindrome made from the product of two 3-digit numbers which is less than N, where 101101 < N < 1000000.

How can I make this more efficient? My solution keeps timing out.

Here is the Code:

# Find the Largest palindrome made from two three digit numbers which are less than N
# Get the input and store it in an array
def get_input():
 limit = int(input())
 limits = []
 for _ in range(limit):
 limits.append(int(input().strip()))
 return limits
# Check if palindrome or not
def is_palindrome(num):
 if str(num) == str(num)[::-1]:
 return True
 else:
 return False
# Find the Largest Palindrome
def largest_palindrome(num):
 largest = 0
 for i in range(2,999+1):
 for j in range(i+1 , 999+1):
 prod = i * j
 if is_palindrome(prod) and prod > largest and prod < num:
 largest = prod
 return largest
# Get the Limits
limits = get_input()
for limit in limits:
 print(largest_palindrome(int(limit)))

Answer 1

Look at my previous answer

• palindrome is divisible by 11
• i and j can be only odd
• reverse loops 999 -> 1 and 999 -> i - 1 - first found palindrome less than n is our palindrome

Simplify code

str(num) is expensive, do it once

def is_palindrome(num):
 snum = str(num)
 return if snum == snum[::-1]

Instead of prod > largest and prod < num You can write num > prod > largest

Use list comprehension

limits = [int(input().strip()) for _ in range(limit)]

• Grajdeanu Alex: To be noted: All palindromes with an even number of digits are divisible by 11.