# Count of distinct Strings possible by swapping prefixes of pairs of Strings from the Array

• Difficulty Level : Medium
• Last Updated : 20 Nov, 2020

Given an array string[] consisting of N numeric strings of length M, the task is to find the number of distinct strings that can be generated by selecting any two strings, say i and j from the array and swap all possible prefixes between them.
Note: Since the answer can be very large, print modulo 1000000007.

Examples:

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Input: N = 2 M = 3 string[] = {“112”, “211”}
Output:
Explanation:
Swapping “1” and “2” between the strings generates “212” and “111“.
Swapping “11” and “21” between the strings generates “212” and “111”.
Swapping “112” and “211” between the strings generates “211” and “112“.
Therefore, 4 distinct strings are generated.

Input: N = 4 M = 5 string[] = {“12121”, “23545”, “11111”, “71261”}
Output: 216

Approach: Considering a string s of the form s1s2s3s4…sm, where s1 is the first letter of any of the strings in the array, s2 is the second letter of any of the strings, and so on, the answer to the problem is the product of count(i) where count(i) is the count of different letters placed at same index in the given strings.

Below is the implementation of the above approach:

## C++

 `// C++ program to implement``// the above approach``#include   ``using` `namespace` `std;` `int` `mod = 1000000007;` `// Function to count the distinct strings``// possible after swapping the prefixes``// between two possible strings of the array``long` `countS(string str[],``int` `n, ``int` `m)``{``    ` `    ``// Stores the count of unique``    ``// characters for each index``    ``unordered_map<``int``,``    ``unordered_set<``char``>> counts;` `    ``for``(``int` `i = 0; i < n; i++)``    ``{``        ` `        ``// Store current string``        ``string s = str[i];``    ` `        ``for``(``int` `j = 0; j < m; j++)``        ``{``            ``counts[j].insert(s[j]);``        ``}``    ``}` `    ``// Stores the total number of``    ``// distinct strings possible``    ``long` `result = 1;``    ` `    ``for``(``auto` `index : counts)``    ``{``        ``result = (result *``                  ``counts[index.first].size()) % mod;``    ``}``    ` `    ``// Return the answer``    ``return` `result;``}` `// Driver Code``int` `main()``{``    ``string str[] = { ``"112"``, ``"211"` `};` `    ``int` `N = 2, M = 3;``    ` `    ``cout << countS(str, N, M);``    ` `    ``return` `0;``}` `// This code is contributed by rutvik_56`

## Java

 `// Java Program to implement``// the above approach``import` `java.util.*;``import` `java.io.*;` `public` `class` `Main {` `    ``static` `int` `mod = ``1000000007``;` `    ``// Function to count the distinct strings``    ``// possible after swapping the prefixes``    ``// between two possible strings of the array``    ``public` `static` `long` `countS(String str[], ``int` `n, ``int` `m)``    ``{` `        ``// Stores the count of unique``        ``// characters for each index``        ``Map > counts``            ``= ``new` `HashMap<>();` `        ``for` `(``int` `i = ``0``; i < m; i++) {``            ``counts.put(i, ``new` `HashSet<>());``        ``}` `        ``for` `(``int` `i = ``0``; i < n; i++) {` `            ``// Store current string``            ``String s = str[i];` `            ``for` `(``int` `j = ``0``; j < m; j++) {``                ``counts.get(j).add(s.charAt(j));``            ``}``        ``}` `        ``// Stores the total number of``        ``// distinct strings possible``        ``long` `result = ``1``;` `        ``for` `(``int` `index : counts.keySet())``            ``result = (result``                    ``* counts.get(index).size())``                    ``% mod;` `        ``// Return the answer``        ``return` `result;``    ``}` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{` `        ``String str[] = { ``"112"``, ``"211"` `};` `        ``int` `N = ``2``, M = ``3``;``        ``System.out.println(countS(str, N, M));``    ``}``}`

## Python3

 `# Python3 program to implement``# the above approach``from` `collections ``import` `defaultdict` `mod ``=` `1000000007` `# Function to count the distinct strings``# possible after swapping the prefixes``# between two possible strings of the array``def` `countS(string: ``list``, n: ``int``, m: ``int``) ``-``> ``int``:``    ` `    ``# Stores the count of unique``    ``# characters for each index``    ``counts ``=` `defaultdict(``lambda``: ``set``())` `    ``for` `i ``in` `range``(n):``        ` `        ``# Store current string``        ``s ``=` `string[i]` `        ``for` `j ``in` `range``(m):``            ``counts[j].add(s[j])` `    ``# Stores the total number of``    ``# distinct strings possible``    ``result ``=` `1` `    ``for` `index ``in` `counts:``        ``result ``=` `(result ``*``                  ``len``(counts[index])) ``%` `mod` `    ``# Return the answer``    ``return` `result` `# Driver Code``if` `__name__ ``=``=` `"__main__"``:``    ` `    ``string ``=` `[ ``"112"``, ``"211"` `]` `    ``N ``=` `2``    ``M ``=` `3``    ` `    ``print``(countS(string, N, M))` `# This code is contributed by sanjeev2552`

## C#

 `// C# Program to implement``// the above approach``using` `System;``using` `System.Collections.Generic;``class` `GFG{` `static` `int` `mod = 1000000007;` `// Function to count the distinct strings``// possible after swapping the prefixes``// between two possible strings of the array``public` `static` `long` `countS(String []str,``                            ``int` `n, ``int` `m)``{` `    ``// Stores the count of unique``    ``// characters for each index``    ``Dictionary<``int``,``    ``HashSet<``char``> > counts = ``new` `Dictionary<``int``,``                                    ``HashSet<``char``>>();` `    ``for` `(``int` `i = 0; i < m; i++)``    ``{``    ``counts.Add(i, ``new` `HashSet<``char``>());``    ``}` `    ``for` `(``int` `i = 0; i < n; i++)``    ``{` `    ``// Store current string``    ``String s = str[i];` `    ``for` `(``int` `j = 0; j < m; j++)``    ``{``        ``counts[j].Add(s[j]);``    ``}``    ``}` `    ``// Stores the total number of``    ``// distinct strings possible``    ``long` `result = 1;` `    ``foreach` `(``int` `index ``in` `counts.Keys)``    ``result = (result * counts[index].Count) % mod;` `    ``// Return the answer``    ``return` `result;``}` `// Driver Code``public` `static` `void` `Main(String[] args)``{``    ``String []str = { ``"112"``, ``"211"` `};` `    ``int` `N = 2, M = 3;``    ``Console.WriteLine(countS(str, N, M));``}``}` `// This code is contributed by Rajput-Ji`
Output:
```4

```

Time Complexity: O(N * M)
Auxiliary Space: O(N)

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