Created section Strings

README.md

@@ -39,6 +39,9 @@ This Repo is For those who want to contribute in an open Source Repo for Hacktob
 |   |── vector.cp
 │   ├── sets-subarray-partition-O(n).cpp
 │   └── stack.cpp  
+├── Strings
+│   ├── Rabin-Karp-Algorithm.cpp
+│   ├── Z-Function.cpp
 ├── Dynamic Programming
 │   ├── basic-fabonic.cpp
 │   ├── binomial-coefficients.cpp

Strings/Rabin-Karp-Algorithm.cpp

@@ -0,0 +1,72 @@
+/*
+    Rabin-Karp Algorithm:
+    source : https://cp-algorithms.com/string/rabin-karp.html
+*/
+
+#include <bits/stdc++.h>
+using namespace std;
+#define int long long
+
+const int BASE = 31, MOD = 1e9 + 7, N = 1e6 + 5;
+
+vector<int> pp(N);
+
+void pre()
+{
+    pp[0] = 1;
+    for (int i = 1; i < N; i++)
+        pp[i] = pp[i - 1] * BASE % MOD;
+}
+
+vector<int> rabin_karp(const string &text, const string &pat)
+{
+    int tn = text.size(), pn = pat.size();
+    vector<int> pref(tn + 1, 0);
+    for (int i = 0; i < tn; i++)
+        pref[i + 1] = (pref[i] + (text[i] - 'a' + 1) * pp[i]) % MOD;
+    int hs = 0;
+    for (int i = 0; i < pn; i++)
+        hs = (hs + (pat[i] - 'a' + 1) * pp[i]) % MOD;
+
+    vector<int> res;
+
+    //checking all substrings of length pattern string
+    for (int i = 0; i + pn - 1 < tn; i++)
+    {
+        int curr = (pref[i + pn] - pref[i] + MOD) % MOD;
+        if (curr == hs * pp[i] % MOD)
+            res.push_back(i);
+    }
+    return res;
+}
+
+signed main()
+{
+    ios::sync_with_stdio(0);
+    cin.tie(0);
+    cout.tie(0);
+
+    pre();
+
+    int T = 1;
+    cin >> T;
+    while (T--)
+    {
+        string text, pat;
+        cin >> text >> pat;
+
+        auto res = rabin_karp(text, pat);
+        if (res.size() == 0)
+            cout << "Not Found\n";
+        else
+        {
+            cout << res.size() << '\n';
+            for (auto &&x : res)
+                cout << ++x << ' ';
+            cout << '\n';
+        }
+        cout << '\n';
+    }
+
+    return 0;
+}

Strings/Z-Function.cpp

@@ -0,0 +1,32 @@
+//Source: https://cp-algorithms.com/string/z-function.html
+
+//basically z[i] = k, means first k characters of string concide with k characters starting from s[i];
+
+/*
+Example :-
+"abacaba" - [0, 0, 1, 0, 3, 0, 1]
+here z[4] = 3; //0 based indexing
+means first 3 characters from s[4] = first 3 character from s[0];
+*/
+
+vector<int> z_function(string &s)
+{
+    int n = s.size();
+    vector<int> z(n);
+    for (int i = 1, l = 0, r = 0; i < n; i++)
+    {
+        if (i <= r)
+            z[i] = min(r - i + 1, z[i - l]);
+        while (i + z[i] < n && s[z[i]] == s[i + z[i]])
+            z[i]++;
+        if (i + z[i] - 1 > r)
+            l = i, r = i + z[i] - 1;
+    }
+    return z;
+}
+
+/*
+Example
+Problem :  https://codeforces.com/contest/432/problem/D
+solution : https://codeforces.com/contest/432/submission/75147854
+*/