Shortest Palindrome — LeetCode 214 Python Solution

The problem

You are given a string s. You can convert s to a palindrome by adding characters in front of it.

Example

Input

s = "aacecaaa"

Output

"aaacecaaa"

Python solution

class Solution:
    def shortestPalindrome(self, s: str) -> str:
        base = 131
        mod = 10**9 + 7
        n = len(s)
        prefix = suffix = 0
        mul = 1
        idx = 0
        for i, c in enumerate(s):
            prefix = (prefix * base + (ord(c) - ord('a') + 1)) % mod
            suffix = (suffix + (ord(c) - ord('a') + 1) * mul) % mod
            mul = (mul * base) % mod
            if prefix == suffix:
                idx = i + 1
        return s if idx == n else s[idx:][::-1] + s

Complexity

Pattern: Hash Map

Trade memory for time: remember what you have seen so the second pass never happens. LeetCode 214. Shortest Palindrome is filed here because the reference solution below belongs to the algorithm family this hub collects, even though its LeetCode tags point elsewhere.

The hash map guide has the Python template for the pattern and the 709 LeetCode problems that use it.

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