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Minimum number of operations required to make two strings equal
  • Difficulty Level : Hard
  • Last Updated : 29 Sep, 2020

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++

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// C++ implementation to find the
// minimum number of operations to
// make two strings equal
 
#include <bits/stdc++.h>
 
#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<pair<char, char> > ans;
 
    // Traversing strings
    for (int i = 0; i < s1.length(); i++) {
        if (s1[i] != s2[i]) {
 
            // If they have differnt 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 << "->"
             <<ans[i].second << endl;
         
}
 
// Driver Code
int main()
{
    // Two strings
    // S1 and S2
    string s1, s2;
    s1 = "abb";
    s2 = "dad";
     
    // Function Call
    minimumOperations(s1, s2);
    return 0;
}

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Java

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// 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<pair> ans = new Vector<pair>();
 
    // Traversing Strings
    for(int i = 0; i < s1.length(); i++)
    {
        if (s1.charAt(i) != s2.charAt(i))
        {
             
            // If they have differnt 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

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Python3

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# 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 differnt 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

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C#

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// 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<pair> ans = new List<pair>();
 
  // Traversing Strings
  for(int i = 0; i < s1.Length; i++)
  {
    if (s1[i] != s2[i])
    {
      // If they have differnt 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

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Output: 

2
a->d
b->a



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