8/29刷题最短回文串

1.最短回文串
给定一个字符串 s，你可以通过在字符串前面添加字符将其转换为回文串。找到并返回可以用这种方式转换的最短回文串。
示例 1:
输入: "aacecaaa"
输出: "aaacecaaa"
示例 2:
输入: "abcd"
输出: "dcbabcd"
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/shortest-palindrome
思路一：字符串哈希

class Solution {
public:
    string shortestPalindrome(string s) {
        int n = s.size();
        int base = 131, mod = 1000000007;
        int left = 0, right = 0, mul = 1;
        int best = -1;
        for (int i = 0; i < n; ++i) {
            left = ((long long)left * base + s[i]) % mod;
            right = (right + (long long)mul * s[i]) % mod;
            if (left == right) {
                best = i;
            }
            mul = (long long)mul * base % mod;
        }
        string add = (best == n - 1 ? "" : s.substr(best + 1, n));
        reverse(add.begin(), add.end());
        return add + s;
    }
};