Find Palindrome With Fixed Length — LeetCode 2217 Python Solution

The problem

Given an integer array queries and a positive integer intLength, return an array answer where answer[i] is either the queries[i]th smallest positive palindrome of length intLength or -1 if no such palindrome exists. A palindrome is a number that reads the same backwards and forwards.

Example

Input

queries = [1,2,3,4,5,90], intLength = 3

Output

[101,111,121,131,141,999]

Explanation

The first few palindromes of length 3 are:

Python solution

class Solution:
    def kthPalindrome(self, queries: List[int], intLength: int) -> List[int]:
        l = (intLength + 1) >> 1
        start, end = 10 ** (l - 1), 10**l - 1
        ans = []
        for q in queries:
            v = start + q - 1
            if v > end:
                ans.append(-1)
                continue
            s = str(v)
            s += s[::-1][intLength % 2 :]
            ans.append(int(s))
        return ans

Complexity

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 2217. Find Palindrome With Fixed Length is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Math.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

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