Find all the duplicate strings and divide them into groups

Here Use Hashing Technique.

/*

Input : 

Enter Number Of String :: 6

Enter String ::

aubdur

rob

anik

aubdur

rob

anik

Output : 

Groups Are ::

Group 1 :

rob

rob

Group 2 :

anik

anik

Group 3 :

aubdur

aubdur

*/

#include<bits/stdc++.h>

#define ll long long int

#define pii pair<ll, int>

#define vector_2D vector<vector<int>>

using namespace std;

ll hash_function(string s)

{

ll p = 31;

ll m = 1e9 + 9;

ll hash_value = 0;

ll p_power = 1;

for(char c : s)

{

hash_value = (hash_value + (c - 'a' + 1) * p_power) % m;

p_power = (p_power * p) % m;

}

return hash_value;

}

vector_2D grouping_string(vector<string> s)

{

int n = s.size();

vector<pii>Hash(n);

for(int i=0;i<n;i++)

Hash[i] = {hash_function(s[i]), i};

sort(Hash.begin(), Hash.end());

vector_2D group;

for(int i=0;i<n;i++)

{

if(i == 0 || Hash[i-1].first != Hash[i].first)

group.emplace_back();

group.back().push_back(Hash[i].second);

}

return group;

}

int main()

{

int n;

cout << "Enter Number Of String :: ";

cin >> n;

vector<string>s(n);

cout << "\nEnter String :: \n";

for(int i=0;i<n;i++)

cin >> s[i];

vector_2D group = grouping_string(s);

cout << "\nGroups Are :: \n";

for(int i=0;i<group.size();i++)

{

cout << "\nGroup " << i+1 << " : \n";

for(int j=0;j<group[i].size();j++)

{

int indx = group[i][j];

cout << s[indx] << "\n";

}

}

return 0;

}