Merge pull request codemistic#484 from Aman5989/patch-19

Create palindrome-substring-queries.cpp

Author: gantavyamalviya

Hashtable/Palindrome-Substring-Queries.cpp

@@ -0,0 +1,173 @@
+/* A C++ program to answer queries to check whether
+the substrings are palindrome or not efficiently */
+#include <bits/stdc++.h>
+using namespace std;
+
+#define p 101
+#define MOD 1000000007
+
+// Structure to represent a query. A query consists
+// of (L, R) and we have to answer whether the substring
+// from index-L to R is a palindrome or not
+struct Query {
+int L, R;
+};
+
+// A function to check if a string str is palindrome
+// in the range L to R
+bool isPalindrome(string str, int L, int R)
+{
+// Keep comparing characters while they are same
+while (R > L)
+if (str[L++] != str[R--])
+return (false);
+return (true);
+}
+
+// A Function to find pow (base, exponent) % MOD
+// in log (exponent) time
+unsigned long long int modPow(
+unsigned long long int base,
+unsigned long long int exponent)
+{
+if (exponent == 0)
+return 1;
+if (exponent == 1)
+return base;
+
+unsigned long long int temp = modPow(base, exponent / 2);
+
+if (exponent % 2 == 0)
+return (temp % MOD * temp % MOD) % MOD;
+else
+return (((temp % MOD * temp % MOD) % MOD)
+* base % MOD)
+% MOD;
+}
+
+// A Function to calculate Modulo Multiplicative Inverse of 'n'
+unsigned long long int findMMI(unsigned long long int n)
+{
+return modPow(n, MOD - 2);
+}
+
+// A Function to calculate the prefix hash
+void computePrefixHash(
+string str, int n,
+unsigned long long int prefix[],
+unsigned long long int power[])
+{
+prefix[0] = 0;
+prefix[1] = str[0];
+
+for (int i = 2; i <= n; i++)
+prefix[i] = (prefix[i - 1] % MOD
++ (str[i - 1] % MOD
+* power[i - 1] % MOD)
+% MOD)
+% MOD;
+
+return;
+}
+
+// A Function to calculate the suffix hash
+// Suffix hash is nothing but the prefix hash of
+// the reversed string
+void computeSuffixHash(
+string str, int n,
+unsigned long long int suffix[],
+unsigned long long int power[])
+{
+suffix[0] = 0;
+suffix[1] = str[n - 1];
+
+for (int i = n - 2, j = 2; i >= 0 && j <= n; i--, j++)
+suffix[j] = (suffix[j - 1] % MOD
++ (str[i] % MOD
+* power[j - 1] % MOD)
+% MOD)
+% MOD;
+return;
+}
+
+// A Function to answer the Queries
+void queryResults(string str, Query q[], int m, int n,
+unsigned long long int prefix[],
+unsigned long long int suffix[],
+unsigned long long int power[])
+{
+for (int i = 0; i <= m - 1; i++) {
+int L = q[i].L;
+int R = q[i].R;
+
+// Hash Value of Substring [L, R]
+unsigned long long hash_LR
+= ((prefix[R + 1] - prefix[L] + MOD) % MOD
+* findMMI(power[L]) % MOD)
+% MOD;
+
+// Reverse Hash Value of Substring [L, R]
+unsigned long long reverse_hash_LR
+= ((suffix[n - L] - suffix[n - R - 1] + MOD) % MOD
+* findMMI(power[n - R - 1]) % MOD)
+% MOD;
+
+// If both are equal then
+// the substring is a palindrome
+if (hash_LR == reverse_hash_LR) {
+if (isPalindrome(str, L, R) == true)
+printf("The Substring [%d %d] is a "
+"palindrome\n",
+L, R);
+else
+printf("The Substring [%d %d] is not a "
+"palindrome\n",
+L, R);
+}
+
+else
+printf("The Substring [%d %d] is not a "
+"palindrome\n",
+L, R);
+}
+
+return;
+}
+
+// A Dynamic Programming Based Approach to compute the
+// powers of 101
+void computePowers(unsigned long long int power[], int n)
+{
+// 101^0 = 1
+power[0] = 1;
+
+for (int i = 1; i <= n; i++)
+power[i] = (power[i - 1] % MOD * p % MOD) % MOD;
+
+return;
+}
+
+/* Driver program to test above function */
+int main()
+{
+string str = "abaaabaaaba";
+int n = str.length();
+
+// A Table to store the powers of 101
+unsigned long long int power[n + 1];
+
+computePowers(power, n);
+
+// Arrays to hold prefix and suffix hash values
+unsigned long long int prefix[n + 1], suffix[n + 1];
+
+// Compute Prefix Hash and Suffix Hash Arrays
+computePrefixHash(str, n, prefix, power);
+computeSuffixHash(str, n, suffix, power);
+
+Query q[] = { { 0, 10 }, { 5, 8 }, { 2, 5 }, { 5, 9 } };
+int m = sizeof(q) / sizeof(q[0]);
+
+queryResults(str, q, m, n, prefix, suffix, power);
+return (0);
+}