# Time:  O(l)
# Space: O(l)

# Given an integer n, find the closest integer (not including itself), which is a palindrome.
#
# The 'closest' is defined as absolute difference minimized between two integers.
#
# Example 1:
# Input: "123"
# Output: "121"
# Note:
# The input n is a positive integer represented by string, whose length will not exceed 18.
# If there is a tie, return the smaller one as answer.

class Solution(object):
    def nearestPalindromic(self, n):
        """
        :type n: str
        :rtype: str
        """
        l = len(n)
        candidates = set((str(10**l + 1), str(10**(l - 1) - 1)))
        prefix = int(n[:(l + 1)/2])
        for i in map(str, (prefix-1, prefix, prefix+1)):
            candidates.add(i + [i, i[:-1]][l%2][::-1])
        candidates.discard(n)
        return min(candidates, key=lambda x: (abs(int(x) - int(n)), int(x)))

Create find-the-closest-palindrome.py