2156. Find Substring With Given Hash Value

The hash of a 0-indexed string s of length k, given integers p and m, is computed using the following function:

• hash(s, p, m) = (val(s[0]) p0 + val(s[1]) p1 + … + val(s[k-1]) * pk-1) mod m.
  Where val(s[i]) represents the index of s[i] in the alphabet from val(‘a’) = 1 to val(‘z’) = 26.

You are given a string s and the integers power, modulo, k, and hashValue. Return sub, the first substring of s of length k such that hash(sub, power, modulo) == hashValue.

The test cases will be generated such that an answer always exists.

A substring is a contiguous non-empty sequence of characters within a string.

class Solution {
public:
    string subStrHash(string s, int power, int modulo, int k, int hashValue) {
        long hash = 0, p = power, pos = 0;

        for(int i = s.length() - 1; i >= 0; --i) {
            hash = (hash * power + s[i] - 'a' + 1) % modulo;
            if(i <= s.length() - k) {
                if(i < s.length() - k) hash = (hash + modulo - (((s[i + k] - 'a' + 1) * p) % modulo)) % modulo;
                if(hash == hashValue) pos = i;
            } else {
                p = (p * power) % modulo;
            }
        }

        return s.substr(pos, k);
    }
};