链接：

1392. 最长快乐前缀

题解：

class Solution {
public:
    string longestPrefix(string s) {
        if (s.size() <= 0) {
            return "";
        }
        int MOD = 1e9 + 7;
        // 构建26的n次方，预处理
        std::vector<long> pow26(s.size());
        pow26[0] = 1;
        for (int i = 1; i < pow26.size(); ++i) {
            pow26[i] = (pow26[i-1] * 26)%MOD;
        }
        // 构建前向hash
        std::vector<long> prefix_hash(s.size());
        prefix_hash[0] = s[0] - 'a';
        for (int i = 1; i < prefix_hash.size(); ++i) {
            prefix_hash[i] = (prefix_hash[i-1] * 26 + (s[i]- 'a')) % MOD;
        }
        // 构建后面的hash
        std::vector<int> post_hash(s.size());
        post_hash[s.size()-1] = s[s.size()-1] - 'a';
        for (int i = s.size()-2; i >= 0; --i) {
            post_hash[i] = ((s[i] - 'a') * pow26[s.size()-i-1] + post_hash[i+1])%MOD;
        }
        for (int len = s.size()-1; len >= 1; --len) {
            // 判断字符串是否相等
            if (prefix_hash[len-1] == post_hash[s.size()-len] &&
                    equal(s, 0, len-1 , s.size() - len, s.size()-1)) {
                return s.substr(0, len);
            }
        }
        return "";
    }
private:
    bool equal(const std::string& a, int l1, int r1, int l2, int r2) {
        for (; l1 <= r1 && l2 <= r2; ++l1, ++l2) {
            if (a[l1] != a[l2]) {
                return false;
            }
        }
        return true;
    }
};