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Count Distinct Strings present in an array using Polynomial rolling hash function
  • Last Updated : 18 Dec, 2020

Given an array of strings arr[], the task is to find the count of distinct strings present in the array using polynomial rolling hash function.

Examples:

Input: arr[] = { “abcde”, “abcce”, “abcdf”, “abcde”, “abcdf” } 
Output:
Explanation: 
Distinct strings in the array are { “abcde”, “abcce”, “abcdf” }. 
Therefore, the required output is 3.

Input: arr[] = { “ab”, “abc”, “abcd”, “abcde”, “a” } 
Output:
Explanation: 
Distinct strings in the array are { “abcde”, “abcd”, “abc”, “ab”, “a” }. 
Therefore, the required output is 5.

Approach: The problem can be solved using Hashing. The idea is to use rolling hash function to calculate the hash value of all the strings of the array and store it in another array, say Hash[]. Finally, print the count of distinct elements in Hash[] array. Follow the steps below to solve the problem:



  • Initialize an array, say Hash[], to store the hash value of all the strings present in the array using rolling hash function.
  • Initialize a variable, say cntElem, to store the count of distinct strings present in the array.
  • Traverse the array arr[]. For every string encountered, calculate the hash value of that string and store it in the hash[] array.
  • Sort the array hash[].
  • Traverse the array hash[]. For every array element, check if hash[i] and hash[i – 1] are equal or not. If found to be false, then increment cntElem by 1.
  • Finally, print the value of cntElem.

Below is the implementation of the above approach:

CPP




// C++ program to implement
// the above approach
 
#include
using namespace std;
 
// Function to find the hash value
// of a string
long long compute_hash(string str)
{
 
    int p = 31;
    int MOD = 1e9 + 9;
    long long hash_val = 0;
    long long mul = 1;
 
    // Traverse the string
    for (char ch : str) {
 
        // Update hash_val
        hash_val
            = (hash_val + (ch - 'a' + 1) * mul)
              % MOD;
 
        // Update mul
        mul = (mul * p) % MOD;
    }
 
    // Return hash_val of str
    return hash_val;
}
 
// Function to find the count of distinct
// strings present in the given array
int distinct_str(vector& arr, int n)
{
    // Store the hash values of
    // the strings
    vector hash(n);
 
    // Traverse the array
    for (int i = 0; i < n; i++) {
 
        // Stores hash value of arr[i]
        hash[i] = compute_hash(arr[i]);
    }
 
    // Sort hash[] array
    sort(hash.begin(), hash.end());
 
    // Stores count of distinct
    // strings in the array
    int cntElem = 1;
 
    // Traverse hash[] array
    for (int i = 1; i < n; i++) {
        if (hash[i] != hash[i - 1]) {
 
            // Update cntElem
            cntElem++;
        }
    }
 
    return cntElem;
}
 
// Driver Code
int main()
{
    vector arr{ "abcde", "abcce",
                        "abcdf", "abcde" };
 
    int N = arr.size();
 
    cout << distinct_str(arr, N) << endl;
 
    return 0;
}

Java




// Java program to implement
// the above approach
import java.util.Arrays;  
 
public class GFG {
     
    // Function to find the hash value
    // of a string
    static int compute_hash(String str)
    {
     
        int p = 31;
        int MOD = (int)1e9 + 9;
        int  hash_val = 0;
        int mul = 1;
     
        // Traverse the string
        for (int i = 0; i < str.length(); i++) {
     
            char ch = str.charAt(i);
             
            // Update hash_val
            hash_val
                = (hash_val + (ch - 'a' + 1) * mul)
                  % MOD;
     
            // Update mul
            mul = (mul * p) % MOD;
        }
     
        // Return hash_val of str
        return hash_val;
    }
     
    // Function to find the count of distinct
    // strings present in the given array
    static int distinct_str(String arr[], int n)
    {
        // Store the hash values of
        // the strings
        int hash[] = new int [n];
     
        // Traverse the array
        for (int i = 0; i < n; i++) {
     
            // Stores hash value of arr[i]
            hash[i] = compute_hash(arr[i]);
        }
     
        // Sort hash[] array
        Arrays.sort(hash);
     
        // Stores count of distinct
        // strings in the array
        int cntElem = 1;
     
        // Traverse hash[] array
        for (int i = 1; i < n; i++) {
            if (hash[i] != hash[i - 1]) {
     
                // Update cntElem
                cntElem++;
            }
        }
     
        return cntElem;
    }
 
    // Driver Code
    public static void main (String[] args)
    {
        String arr[] = { "abcde", "abcce",
                            "abcdf", "abcde" };
     
        int N = arr.length;
     
        System.out.println(distinct_str(arr, N));   
    }
}
 
// This code is contributed by AnkThon

Python3




# Python3 program to implement
# the above approach
 
# Function to find the hash value
# of a
def compute_hash(str):
 
    p = 31
    MOD = 10**9 + 9
    hash_val = 0
    mul = 1
 
    # Traverse the
    for ch in str:
 
        # Update hash_val
        hash_val = (hash_val + (ord(ch) - ord('a') + 1) * mul) % MOD
 
        # Update mul
        mul = (mul * p) % MOD
 
    # Return hash_val of str
    return hash_val
 
# Function to find the count of distinct
# strings present in the given array
def distinct_str(arr, n):
   
    # Store the hash values of
    # the strings
    hash = [0]*(n)
 
    # Traverse the array
    for i in range(n):
 
        # Stores hash value of arr[i]
        hash[i] = compute_hash(arr[i])
 
    # Sort hash[] array
    hash = sorted(hash)
 
    # Stores count of distinct
    # strings in the array
    cntElem = 1
 
    # Traverse hash[] array
    for i in range(1, n):
        if (hash[i] != hash[i - 1]):
 
            # Update cntElem
            cntElem += 1
   
    return cntElem
 
# Driver Code
if __name__ == '__main__':
    arr=["abcde", "abcce","abcdf", "abcde"]
 
    N = len(arr)
 
    print(distinct_str(arr, N))
 
# This code is contributed by mohit kumar 29

C#




// C# program to implement
// the above approach
using System;
 
class GFG
{
     
    // Function to find the hash value
    // of a string
    static int compute_hash(string str)
    {
     
        int p = 31;
        int MOD = (int)1e9 + 9;
        int  hash_val = 0;
        int mul = 1;
     
        // Traverse the string
        for (int i = 0; i < str.Length; i++)
        {   
            char ch = str[i];
             
            // Update hash_val
            hash_val = (hash_val + (ch -
                        'a' + 1) * mul) % MOD;
     
            // Update mul
            mul = (mul * p) % MOD;
        }
     
        // Return hash_val of str
        return hash_val;
    }
     
    // Function to find the count of distinct
    // strings present in the given array
    static int distinct_str(string []arr, int n)
    {
       
        // Store the hash values of
        // the strings
        int []hash = new int [n];
     
        // Traverse the array
        for (int i = 0; i < n; i++)
        {
     
            // Stores hash value of arr[i]
            hash[i] = compute_hash(arr[i]);
        }
     
        // Sort hash[] array
        Array.Sort(hash);
     
        // Stores count of distinct
        // strings in the array
        int cntElem = 1;
     
        // Traverse hash[] array
        for (int i = 1; i < n; i++)
        {
            if (hash[i] != hash[i - 1])
            {
     
                // Update cntElem
                cntElem++;
            }
        }   
        return cntElem;
    }
 
    // Driver Code
    public static void Main (String[] args)
    {
        string []arr = { "abcde", "abcce",
                            "abcdf", "abcde" }; 
        int N = arr.Length; 
        Console.WriteLine(distinct_str(arr, N));   
    }
}
 
// This code is contributed by AnkThon
Output: 
3

 

Time Complexity: O(N * M), where M is the maximum length of the string 
Auxiliary Space: O(N)

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