Rabin Karp Algorithm added

C-Plus-Plus/README.md

@@ -201,5 +201,6 @@ _add list here_
 - [Kth largest element](other/Kth_largest_element.cpp.cpp)
 - [Kth smallest element](other/Kth_smallest_element.cpp)
 - [Generate all Subsets](other/subsets.cpp)
-- [Stock Span Problem](other/Stock_span_problem.cpp)
 - [Boyer–Moore majority vote algorithm](other/majority_vote_algorithm.cpp)
+- [Rabin Karp Algorithm (using rolling hash)](other/Rabin_Karp_Algorithm_using_rolling_hash.cpp)
+- [Rabin Karp Algorithm (using prefix sum)](other/Rabin_Karp_Algorithm_using_prefix_sum.cpp)

C-Plus-Plus/other/Rabin_Karp_Algorithm_using_prefix_sum.cpp

@@ -0,0 +1,123 @@
+/*
+Rabin Karp Algorithm (using prefix sum)
+=========================================
+Given a text and a pattern, find all the occurences of the pattern in the text
+
+Time Complexity: O(n) n=size of string
+Space Complexity: O(1)
+*/
+#include <bits/stdc++.h>
+#define int long long
+using namespace std;
+
+const int mod = 1e9 + 7;
+const int p = 31;
+
+// returns (a^b)%mod
+int powr(int a, int b) {
+	int ans = 1;
+	while (b) {
+		if (b & 1LL) {
+			ans *= a;
+			ans %= mod;
+		}
+		a *= a;
+		b = b >> 1;
+		a %= mod;
+	}
+	return ans % mod;
+}
+
+// returns (a^-1)%mod
+int inv(int a) {
+	return powr(a, mod - 2) % mod;
+}
+
+// Assigns hash to a string
+int polyHashString(string s) {
+	int hash = 0;
+	int coef = 1;
+	for (int i = 0; i < s.size(); i++) {
+		hash += (s[i] - 'a' + 1) * coef;
+		coef *= p;
+		coef %= mod;
+		hash %= mod;
+	}
+	return hash % mod;
+}
+
+// Rabin Karp Algorithm for pattern matching
+// returns indices of pattern in string s
+void rabinKarp(string s, string pat) {
+
+	int pat_hash = polyHashString(pat);
+	int n = s.size(), m = pat.size();
+
+	int pref[n] = {0};
+
+	for (int i = 0; i < n; i++) {
+		pref[i] = (s[i] - 'a' + 1) * powr(p, i);
+		pref[i] %= mod;
+	}
+
+	for (int i = 1; i < n; i++) {
+		pref[i] += pref[i - 1];
+		pref[i] %= mod;
+	}
+
+	bool patFound = 0;
+	for (int i = 0; i <= (n - m); i++) {
+		// substring from s[l.......r]
+		// r = i+m - 1
+		int l = i;
+		int r = i + m - 1;
+
+		int new_hash = pref[r];
+
+		if (i - 1 >= 0) {
+			new_hash -= pref[l - 1];
+		}
+
+		new_hash += mod; new_hash %= mod;
+
+		if (new_hash == (pat_hash * powr(p, l)) % mod) {
+			cout << i << endl;
+			patFound = 1;
+		}
+	}
+
+	if (!patFound) {
+		cout << "Not found";
+	}
+}
+
+// Driver Code
+signed main() {
+
+	// Take input
+	string s, pat;
+	cin >> s >> pat;
+
+	rabinKarp(s, pat);
+}
+
+/*
+Input:
+ilovecoding
+love
+
+Output:
+1
+
+Input:
+aaaaaaaa
+aaa
+
+Output:
+0
+1
+2
+3
+4
+5
+*/

C-Plus-Plus/other/Rabin_Karp_Algorithm_using_rolling_hash.cpp

@@ -0,0 +1,129 @@
+/*
+Rabin Karp Algorithm (using rolling hash)
+=========================================
+Given a text and a pattern, find all the occurences of the pattern in the text
+
+Time Complexity: O(n) n=size of string
+Space Complexity: O(1)
+*/
+#include <bits/stdc++.h>
+#define int long long
+using namespace std;
+
+const int mod = 1e9 + 7;
+const int p = 31;
+
+// returns (a^b)%mod
+int powr(int a, int b) {
+	int ans = 1;
+	while (b) {
+		if (b & 1LL) {
+			ans *= a;
+			ans %= mod;
+		}
+		a *= a;
+		b = b >> 1;
+		a %= mod;
+	}
+	return ans % mod;
+}
+
+// returns (a^-1)%mod
+int inv(int a) {
+	return powr(a, mod - 2) % mod;
+}
+
+// Assigns hash to a string
+int polyHashString(string s) {
+	int hash = 0;
+	int coef = 1;
+	for (int i = 0; i < s.size(); i++) {
+		hash += (s[i] - 'a' + 1) * coef;
+		coef *= p;
+		coef %= mod;
+		hash %= mod;
+	}
+	return hash % mod;
+}
+
+// Rabin Karp Algorithm for pattern matching
+// returns indices of pattern in string s
+void rabinKarp(string s, string pat) {
+
+	int n = s.size();
+	int m = pat.size();
+
+	int patHash = polyHashString(pat);
+	// Maintains hash of current window of size m
+	int currHash = polyHashString(s.substr(0, m));
+
+	bool patFound = 0;
+
+	// If first window matches pattern
+	if (currHash == patHash) {
+		cout << 0 << endl;
+		patFound = 1;
+	}
+
+	// coefficient for next character
+	int coef = powr(p, m - 1);
+
+	for (int i = 1; i <= (n - m); i++) {
+		int newHash = currHash;
+
+		// Subtract previous character
+		newHash -= s[i - 1] - 'a' + 1;
+		newHash += mod; newHash %= mod;
+
+		// divide by prime
+		newHash *= inv(p);
+		newHash %= mod;
+
+		// Add next char
+		newHash += (s[i + m - 1] - 'a' + 1) * coef;
+		newHash %= mod;
+
+		// Check
+		if (newHash == patHash) {
+			cout << i << endl;
+			patFound = 1;
+		}
+
+		currHash = newHash;
+	}
+
+	if (!patFound) {
+		cout << "Not found";
+	}
+}
+
+// Driver Code
+signed main() {
+
+	// Take input
+	string s, pat;
+	cin >> s >> pat;
+
+	rabinKarp(s, pat);
+}
+
+/*
+Input:
+ilovecoding
+love
+
+Output:
+1
+
+Input:
+aaaaaaaa
+aaa
+
+Output:
+0
+1
+2
+3
+4
+5
+*/