Find the Closest Palindrome — LeetCode 564 Python Solution
HardMathString
- Problem
- #564
- Pattern
- Math and Number Theory
- Reading time
- 4 min
- Source
- leetcode.com
The problem
Given a string n representing an integer, return the closest integer (not including itself), which is a palindrome. If there is a tie, return the smaller one.
Example
- Input
- n = "123"
- Output
- "121"
Python solution
Python
class Solution:
def nearestPalindromic(self, n: str) -> str:
x = int(n)
l = len(n)
res = {10 ** (l - 1) - 1, 10**l + 1}
left = int(n[: (l + 1) >> 1])
for i in range(left - 1, left + 2):
j = i if l % 2 == 0 else i // 10
while j:
i = i * 10 + j % 10
j //= 10
res.add(i)
res.discard(x)
ans = -1
for t in res:
if (
ans == -1
or abs(t - x) < abs(ans - x)
or (abs(t - x) == abs(ans - x) and t < ans)
):
ans = t
return str(ans)Complexity
| Measure | Complexity |
|---|---|
| Time | O(n) or O(1) |
| Space | O(1) auxiliary |
Pattern: Math and Number Theory
Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 564. Find the Closest Palindrome 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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Frequently asked questions
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- LeetCode 564. Find the Closest Palindrome is tagged Math and String on LeetCode.