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Minimum number of operations required to make two strings equal
• Difficulty Level : Hard
• Last Updated : 21 May, 2021

Given Two Strings s1 and s2 containing only lowercase letters of same length. The task is to make these strings equal by using the minimum number of operations. In one operation you can equalize any letter to any other alphabet.

Examples:

Input: S1 = “abb”, S2 = “dad”
Output:
a -> d
b -> a
Explanation:
Strings after the first operation –
S1 = “dbb”, S2 = “ddd”
Strings after the second operation –
S1 = “ddd”, S2 = “ddd”

Input: S1 = “bab”, S2 = “aab”
Output:
b -> a
Explanation:
Strings after the first operation –
S1 = “aaa”, S2 = “aaa”

Approach: The idea is to use the Disjoint set union data structure. Below is the illustration of the steps:

• Initialize the parent of each alphabet to itself.
• Traverse the two strings simultaneously with the help of index and check if the corresponding characters have different parents then merge the string which has less rank in the strings.

Below is the implementation of the above approach:

## C++

 `// C++ implementation to find the``// minimum number of operations to``// make two strings equal` `#include ` `#define MAX 500001``using` `namespace` `std;` `int` `parent[MAX];``int` `Rank[MAX];` `// Function to find out``// parent of an alphabet``int` `find(``int` `x)``{``    ``return` `parent[x] =``       ``parent[x] == x ? x :``            ``find(parent[x]);``}` `// Function to merge two``// different alphabets``void` `merge(``int` `r1, ``int` `r2)``{``    ``// Merge a and b using``    ``// rank compression``    ``if` `(r1 != r2) {``        ``if` `(Rank[r1] > Rank[r2]) {``            ``parent[r2] = r1;``            ``Rank[r1] += Rank[r2];``        ``}``        ``else` `{``            ``parent[r1] = r2;``            ``Rank[r2] += Rank[r1];``        ``}``    ``}``}` `// Function to find the minimum``// number of operations required``void` `minimumOperations(string s1,``                       ``string s2){``    ` `    ``// Initializing parent to i``    ``// and rank(size) to 1``    ``for` `(``int` `i = 1; i <= 26; i++) {``        ``parent[i] = i;``        ``Rank[i] = 1;``    ``}``    ` `    ``// We will store our``    ``// answerin this vector``    ``vector > ans;` `    ``// Traversing strings``    ``for` `(``int` `i = 0; i < s1.length(); i++) {``        ``if` `(s1[i] != s2[i]) {` `            ``// If they have different parents``            ``if` `(find(s1[i] - 96) !=``                ``find(s2[i] - 96)) {``                ` `                ``// Find their respective``                ``// parents and merge them``                ``int` `x = find(s1[i] - 96);``                ``int` `y = find(s2[i] - 96);``                ``merge(x, y);` `                ``// Store this in``                ``// our Answer vector``                ``ans.push_back({ s1[i], s2[i] });``            ``}``        ``}``    ``}` `    ``// Number of operations``    ``cout << ans.size() << endl;``    ``for` `(``int` `i = 0; i < ans.size(); i++)``        ``cout << ans[i].first << ``"->"``             ``<

## Java

 `// Java implementation to find the``// minimum number of operations to``// make two Strings equal``import` `java.util.*;` `class` `GFG{``    ` `static` `final` `int` `MAX = ``500001``;``static` `int` `[]parent = ``new` `int``[MAX];``static` `int` `[]Rank = ``new` `int``[MAX];` `static` `class` `pair``{``    ``char` `first, second;``    ` `    ``public` `pair(``char` `first, ``char` `second)``    ``{``        ``this``.first = first;``        ``this``.second = second;``    ``}``}` `// Function to find out``// parent of an alphabet``static` `int` `find(``int` `x)``{``    ``return` `parent[x] = parent[x] == x ? x :``                  ``find(parent[x]);``}` `// Function to merge two``// different alphabets``static` `void` `merge(``int` `r1, ``int` `r2)``{``    ` `    ``// Merge a and b using``    ``// rank compression``    ``if` `(r1 != r2)``    ``{``        ``if` `(Rank[r1] > Rank[r2])``        ``{``            ``parent[r2] = r1;``            ``Rank[r1] += Rank[r2];``        ``}``        ``else``        ``{``            ``parent[r1] = r2;``            ``Rank[r2] += Rank[r1];``        ``}``    ``}``}` `// Function to find the minimum``// number of operations required``static` `void` `minimumOperations(String s1,``                              ``String s2)``{``    ` `    ``// Initializing parent to i``    ``// and rank(size) to 1``    ``for``(``int` `i = ``1``; i <= ``26``; i++)``    ``{``        ``parent[i] = i;``        ``Rank[i] = ``1``;``    ``}``    ` `    ``// We will store our``    ``// answer in this vector``    ``Vector ans = ``new` `Vector();` `    ``// Traversing Strings``    ``for``(``int` `i = ``0``; i < s1.length(); i++)``    ``{``        ``if` `(s1.charAt(i) != s2.charAt(i))``        ``{``            ` `            ``// If they have different parents``            ``if` `(find(s1.charAt(i) - ``96``) !=``                ``find(s2.charAt(i) - ``96``))``            ``{``                ` `                ``// Find their respective``                ``// parents and merge them``                ``int` `x = find(s1.charAt(i) - ``96``);``                ``int` `y = find(s2.charAt(i) - ``96``);``                ``merge(x, y);` `                ``// Store this in``                ``// our Answer vector``                ``ans.add(``new` `pair(s1.charAt(i),``                                 ``s2.charAt(i)));``            ``}``        ``}``    ``}` `    ``// Number of operations``    ``System.out.print(ans.size() + ``"\n"``);``    ``for``(``int` `i = ``0``; i < ans.size(); i++)``        ``System.out.print(ans.get(i).first + ``"->"` `+``                         ``ans.get(i).second +``"\n"``);``}` `// Driver Code``public` `static` `void` `main(String[] args)``{``    ` `    ``// Two Strings``    ``// S1 and S2``    ``String s1, s2;``    ``s1 = ``"abb"``;``    ``s2 = ``"dad"``;``    ` `    ``// Function call``    ``minimumOperations(s1, s2);``}``}` `// This code is contributed by Princi Singh`

## Python3

 `# Python3 implementation to find the``# minimum number of operations to``# make two strings equal` `MAX` `=` `500001``parent ``=` `[``0``] ``*` `MAX``Rank ``=` `[``0``] ``*` `MAX` `# Function to find out``# parent of an alphabet``def` `find(x):``    ` `    ``if` `parent[x] ``=``=` `x:``        ``return` `x``    ``else``:``        ``return` `find(parent[x])` `# Function to merge two``# different alphabets``def` `merge(r1, r2):` `    ``# Merge a and b using``    ``# rank compression``    ``if``(r1 !``=` `r2):``        ``if``(Rank[r1] > Rank[r2]):``            ``parent[r2] ``=` `r1``            ``Rank[r1] ``+``=` `Rank[r2]` `        ``else``:``            ``parent[r1] ``=` `r2``            ``Rank[r2] ``+``=` `Rank[r1]` `# Function to find the minimum``# number of operations required``def` `minimumOperations(s1, s2):` `    ``# Initializing parent to i``    ``# and rank(size) to 1``    ``for` `i ``in` `range``(``1``, ``26` `+` `1``):``        ``parent[i] ``=` `i``        ``Rank[i] ``=` `1` `    ``# We will store our``    ``# answerin this list``    ``ans ``=` `[]` `    ``# Traversing strings``    ``for` `i ``in` `range``(``len``(s1)):``        ``if``(s1[i] !``=` `s2[i]):` `            ``# If they have different parents``            ``if``(find(``ord``(s1[i]) ``-` `96``) !``=``               ``find(``ord``(s2[i]) ``-` `96``)):` `                ``# Find their respective``                ``# parents and merge them``                ``x ``=` `find(``ord``(s1[i]) ``-` `96``)``                ``y ``=` `find(``ord``(s2[i]) ``-` `96``)``                ``merge(x, y)` `                ``# Store this in``                ``# our Answer list``                ``ans.append([s1[i], s2[i]])` `    ``# Number of operations``    ``print``(``len``(ans))``    ``for` `i ``in` `ans:``        ``print``(i[``0``], ``"->"``, i[``1``])``        ` `# Driver code``if` `__name__ ``=``=` `'__main__'``:` `    ``# Two strings``    ``# S1 and S2``    ``s1 ``=` `"abb"``    ``s2 ``=` `"dad"``    ` `    ``# Function Call``    ``minimumOperations(s1, s2)` `# This code is contributed by Shivam Singh`

## C#

 `// C# implementation to find the``// minimum number of operations to``// make two Strings equal``using` `System;``using` `System.Collections.Generic;``class` `GFG{``    ` `static` `readonly` `int` `MAX = 500001;``static` `int` `[]parent = ``new` `int``[MAX];``static` `int` `[]Rank = ``new` `int``[MAX];``class` `pair``{``  ``public` `char` `first, second;``  ``public` `pair(``char` `first, ``char` `second)``  ``{``    ``this``.first = first;``    ``this``.second = second;``  ``}``}` `// Function to find out``// parent of an alphabet``static` `int` `find(``int` `x)``{``  ``return` `parent[x] = parent[x] ==``         ``x ? x : find(parent[x]);``}` `// Function to merge two``// different alphabets``static` `void` `merge(``int` `r1, ``int` `r2)``{``  ``// Merge a and b using``  ``// rank compression``  ``if` `(r1 != r2)``  ``{``    ``if` `(Rank[r1] > Rank[r2])``    ``{``      ``parent[r2] = r1;``      ``Rank[r1] += Rank[r2];``    ``}``    ``else``    ``{``      ``parent[r1] = r2;``      ``Rank[r2] += Rank[r1];``    ``}``  ``}``}` `// Function to find the minimum``// number of operations required``static` `void` `minimumOperations(String s1,``                              ``String s2)``{``  ``// Initializing parent to i``  ``// and rank(size) to 1``  ``for``(``int` `i = 1; i <= 26; i++)``  ``{``    ``parent[i] = i;``    ``Rank[i] = 1;``  ``}` `  ``// We will store our``  ``// answer in this vector``  ``List ans = ``new` `List();` `  ``// Traversing Strings``  ``for``(``int` `i = 0; i < s1.Length; i++)``  ``{``    ``if` `(s1[i] != s2[i])``    ``{``      ``// If they have different parents``      ``if` `(find(s1[i] - 96) !=``          ``find(s2[i] - 96))``      ``{``        ``// Find their respective``        ``// parents and merge them``        ``int` `x = find(s1[i] - 96);``        ``int` `y = find(s2[i] - 96);``        ``merge(x, y);` `        ``// Store this in``        ``// our Answer vector``        ``ans.Add(``new` `pair(s1[i],``                         ``s2[i]));``      ``}``    ``}``  ``}` `  ``// Number of operations``  ``Console.Write(ans.Count + ``"\n"``);``  ``for``(``int` `i = 0; i < ans.Count; i++)``    ``Console.Write(ans[i].first + ``"->"` `+``                  ``ans[i].second + ``"\n"``);``}` `// Driver Code``public` `static` `void` `Main(String[] args)``{   ``  ``// Two Strings``  ``// S1 and S2``  ``String s1, s2;``  ``s1 = ``"abb"``;``  ``s2 = ``"dad"``;` `  ``// Function call``  ``minimumOperations(s1, s2);``}``}` `// This code is contributed by Princi Singh`
Output:
```2
a->d
b->a```

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