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Minimum steps to convert all paths in matrix from top left to bottom right as palindromic paths | Set 2

  • Last Updated : 06 Jul, 2021
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Given a matrix mat[][] with N rows and M columns. The task is to find the minimum number of changes required in the matrix such that every path from top left to bottom right is a palindromic path. In a path only right and bottom movements are allowed from one cell to another cell.

Examples: 

Input: M = 2, N = 2, mat[M][N] = {{0, 0}, {0, 1}} 
Output:
Explanation: 
Change matrix[0][0] from 0 to 1. The two paths from (0, 0) to (1, 1) become palindromic.

Input: M = 3, N = 7, mat[M][N] = {{1, 2, 3, 4, 5, 6, 7}, {2, 2, 3, 3, 4, 3, 2}, {1, 2, 3, 2, 5, 6, 4}} 
Output: 10

Naive Approach: For the Naive Approach please refer to this article. 



Time Complexity: O(N^3) 
Auxiliary Space: O(N)

Efficient Approach: 
The following observations have to be made: 

  • A unique diagonal exists for every value (i+j) where i is the row index and j is the column index.
  • The task reduces to selecting two diagonals, which are at the same distance from cell (0, 0) and cell (M – 1, N – 1) respectively and making all their elements equal to a single number, which is repeated most number of times in both the chosen diagonals.
  • If the number of elements between cell (0, 0) and (M – 1, N – 1) are odd, then a common diagonal equidistant from both the cells exists. Elements of that diagonal need not be modified as they do not affect the palindromic arrangement of a path.
  • If the numbers of cells between (0, 0) and (M – 1, N – 1) are even, no such common diagonal exists.

Follow the below steps to solve the problem:  

  1. Create a 2D array frequency_diagonal that stores the frequency of all numbers in each chosen diagonal.
  2. Each diagonal can be uniquely represented as the sum of (i, j).
  3. Initialize a count variable that stores the count of the total number of cells where the values have to be replaced.
  4. Iterate over the mat[][] and increment the frequency of current element in the diagonal ((i + j) value) where it belongs to.
  5. Initialize a variable number_of_elements to 1, which stores the number of elements in each of the currently chosen pair of diagonals.
  6. Initialize start = 0 and end = M + N – 2 and repeat the steps below until start < end
    • Find the frequency of the number which appears maximum times in the two selected diagonals that are equidistant from (0, 0) and (M-1, N-1).
    • Let the frequency in the above step be X. Add the value (total number of elements in the two diagonals – X) to count the minimum number of changes.
    • Increment start by 1 and decrement end by 1 and if the number of elements in the current diagonal is less than the maximum possible elements in any diagonal of the matrix, then increment number_of_elements by 1.
  7. Print the value of total count after the above steps.

Below is the implementation of the above approach:

C++




// C++ program of the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to calculate the minimum
// number of replacements
int MinReplacements(
    int M, int N, int mat[][30])
{
 
    // 2D array to store frequency
    // of all the numbers in
    // each diagonal
 
    int frequency_diagonal[100][10005];
 
    // Initialise all the elements
    // of 2D array with 0
    memset(frequency_diagonal, 0,
           sizeof(frequency_diagonal));
 
    // Initialise answer as 0
    int answer = 0;
 
    // Iterate over the matrix
    for (int i = 0; i < M; i++) {
        for (int j = 0; j < N; j++) {
 
            // Update the frequency of
            // number mat[i][j]
            // for the diagonal
            // identified by (i+j)
            frequency_diagonal[i + j]
                              [mat[i][j]]++;
        }
    }
 
    // Initialize start as 0
    // which indicates the
    // first diagonal
    int start = 0;
 
    // Initialize end as
    // M + N - 2 which indicates
    // the last diagonal
    int end = M + N - 2;
 
    // Number of elements in
    // the current diagonal
    int no_of_elemnts = 1;
 
    // Maximum possible number
    // of elements in a diagonal
    // can be minimum of (number of
    // rows and number of columns)
    int max_elements = min(M, N);
 
    while (start < end) {
 
        // The frequency of number
        // which occurs for the
        // maximum number of times
        // in the two selected
        // diagonals
        int X = INT_MIN;
 
        for (int i = 0; i <= 10000; i++) {
            X = max(
                X,
                frequency_diagonal[start][i]
                    + frequency_diagonal[end][i]);
        }
 
        answer = answer + (2 * (no_of_elemnts)) - X;
 
        // Increment start
        start++;
        // Decrement end
        end--;
 
        // Increment current number
        // of elements until it reaches
        // the maximum possible value
        if (no_of_elemnts < max_elements)
            no_of_elemnts++;
    }
 
    // return the final answer
    return answer;
}
 
// Driver Code
int main()
{
    // Number of rows
    int M = 3;
 
    // Number of columns
    int N = 7;
 
    int mat[30][30]
        = { { 1, 2, 3, 4, 5, 6, 7 },
            { 2, 2, 3, 3, 4, 3, 2 },
            { 1, 2, 3, 2, 5, 6, 4 } };
 
    cout << MinReplacements(M, N, mat)
         << endl;
}

Java




// Java program of the above approach
class GFG{
 
// Function to calculate the minimum
// number of replacements
static int MinReplacements(int M, int N,
                           int mat[][])
{
     
    // 2D array to store frequency
    // of all the numbers in
    // each diagonal
    int [][]frequency_diagonal = new int[100][10005];
 
    // Initialise answer as 0
    int answer = 0;
 
    // Iterate over the matrix
    for(int i = 0; i < M; i++)
    {
        for(int j = 0; j < N; j++)
        {
 
            // Update the frequency of
            // number mat[i][j]
            // for the diagonal
            // identified by (i+j)
            frequency_diagonal[i + j][mat[i][j]]++;
        }
    }
 
    // Initialize start as 0
    // which indicates the
    // first diagonal
    int start = 0;
 
    // Initialize end as
    // M + N - 2 which indicates
    // the last diagonal
    int end = M + N - 2;
 
    // Number of elements in
    // the current diagonal
    int no_of_elemnts = 1;
 
    // Maximum possible number
    // of elements in a diagonal
    // can be minimum of (number of
    // rows and number of columns)
    int max_elements = Math.min(M, N);
 
    while (start < end)
    {
 
        // The frequency of number
        // which occurs for the
        // maximum number of times
        // in the two selected
        // diagonals
        int X = Integer.MIN_VALUE;
 
        for(int i = 0; i <= 10000; i++)
        {
            X = Math.max(X,
                frequency_diagonal[start][i] +
                frequency_diagonal[end][i]);
        }
 
        answer = answer + (2 * (no_of_elemnts)) - X;
 
        // Increment start
        start++;
         
        // Decrement end
        end--;
 
        // Increment current number
        // of elements until it reaches
        // the maximum possible value
        if (no_of_elemnts < max_elements)
            no_of_elemnts++;
    }
 
    // return the final answer
    return answer;
}
 
// Driver Code
public static void main(String[] args)
{
     
    // Number of rows
    int M = 3;
 
    // Number of columns
    int N = 7;
 
    int mat[][] = { { 1, 2, 3, 4, 5, 6, 7 },
                    { 2, 2, 3, 3, 4, 3, 2 },
                    { 1, 2, 3, 2, 5, 6, 4 } };
 
    System.out.print(MinReplacements(M, N, mat) + "\n");
}
}
 
// This code is contributed by 29AjayKumar

Python3




# Python3 program of the above approach
import sys
 
# Function to calculate the minimum
# number of replacements
def MinReplacements(M, N, mat):
 
    # 2D array to store frequency
    # of all the numbers in
    # each diagonal
    frequency_diagonal = [[0 for x in range(10005)]
                             for y in range (100)];
 
    # Initialise answer as 0
    answer = 0
 
    # Iterate over the matrix
    for i in range(M):
        for j in range(N):
 
            # Update the frequency of
            # number mat[i][j]
            # for the diagonal
            # identified by (i+j)
            frequency_diagonal[i + j][mat[i][j]] += 1
 
    # Initialize start as 0
    # which indicates the
    # first diagonal
    start = 0
 
    # Initialize end as
    # M + N - 2 which indicates
    # the last diagonal
    end = M + N - 2
 
    # Number of elements in
    # the current diagonal
    no_of_elemnts = 1
 
    # Maximum possible number
    # of elements in a diagonal
    # can be minimum of (number of
    # rows and number of columns)
    max_elements = min(M, N)
 
    while (start < end):
 
        # The frequency of number
        # which occurs for the
        # maximum number of times
        # in the two selected
        # diagonals
        X = -sys.maxsize - 1
 
        for i in range(10001):
            X = max(X,
                    frequency_diagonal[start][i] +
                    frequency_diagonal[end][i])
         
        answer = answer + (2 * (no_of_elemnts)) - X
 
        # Increment start
        start += 1
         
        # Decrement end
        end -= 1
 
        # Increment current number
        # of elements until it reaches
        # the maximum possible value
        if (no_of_elemnts < max_elements):
            no_of_elemnts += 1
 
    # Return the final answer
    return answer
 
# Driver Code
 
# Number of rows
M = 3
 
# Number of columns
N = 7
 
mat = [ [ 1, 2, 3, 4, 5, 6, 7 ],
        [ 2, 2, 3, 3, 4, 3, 2 ],
        [ 1, 2, 3, 2, 5, 6, 4 ] ]
 
print(MinReplacements(M, N, mat))
 
# This code is contributed by chitranayal

C#




// C# program of the above approach
using System;
 
class GFG{
 
// Function to calculate the minimum
// number of replacements
static int MinReplacements(int M, int N,
                           int [,]mat)
{
     
    // 2D array to store frequency
    // of all the numbers in
    // each diagonal
    int [,]frequency_diagonal = new int[100, 10005];
 
    // Initialise answer as 0
    int answer = 0;
 
    // Iterate over the matrix
    for(int i = 0; i < M; i++)
    {
        for(int j = 0; j < N; j++)
        {
 
            // Update the frequency of
            // number mat[i,j]
            // for the diagonal
            // identified by (i+j)
            frequency_diagonal[i + j, mat[i, j]]++;
        }
    }
 
    // Initialize start as 0
    // which indicates the
    // first diagonal
    int start = 0;
 
    // Initialize end as
    // M + N - 2 which indicates
    // the last diagonal
    int end = M + N - 2;
 
    // Number of elements in
    // the current diagonal
    int no_of_elemnts = 1;
 
    // Maximum possible number
    // of elements in a diagonal
    // can be minimum of (number of
    // rows and number of columns)
    int max_elements = Math.Min(M, N);
 
    while (start < end)
    {
 
        // The frequency of number
        // which occurs for the
        // maximum number of times
        // in the two selected
        // diagonals
        int X = int.MinValue;
 
        for(int i = 0; i <= 10000; i++)
        {
            X = Math.Max(X,
                frequency_diagonal[start, i] +
                frequency_diagonal[end, i]);
        }
 
        answer = answer + (2 * (no_of_elemnts)) - X;
 
        // Increment start
        start++;
         
        // Decrement end
        end--;
 
        // Increment current number
        // of elements until it reaches
        // the maximum possible value
        if (no_of_elemnts < max_elements)
            no_of_elemnts++;
    }
 
    // Return the readonly answer
    return answer;
}
 
// Driver Code
public static void Main(String[] args)
{
     
    // Number of rows
    int M = 3;
 
    // Number of columns
    int N = 7;
 
    int [,]mat = { { 1, 2, 3, 4, 5, 6, 7 },
                   { 2, 2, 3, 3, 4, 3, 2 },
                   { 1, 2, 3, 2, 5, 6, 4 } };
 
    Console.Write(MinReplacements(M, N, mat) + "\n");
}
}
 
// This code is contributed by amal kumar choubey

Javascript




<script>
 
// Javascript program of the above approach
 
// Function to calculate the minimum
// number of replacements
function MinReplacements(M,N,mat)
{
    // 2D array to store frequency
    // of all the numbers in
    // each diagonal
    let frequency_diagonal = new Array(100);
      for(let i=0;i<100;i++)
    {
        frequency_diagonal[i]=new Array(10005);
        for(let j=0;j<10005;j++)
        {
            frequency_diagonal[i][j]=0;
        }
    }
   
    // Initialise answer as 0
    let answer = 0;
   
    // Iterate over the matrix
    for(let i = 0; i < M; i++)
    {
        for(let j = 0; j < N; j++)
        {
   
            // Update the frequency of
            // number mat[i][j]
            // for the diagonal
            // identified by (i+j)
            frequency_diagonal[i + j][mat[i][j]]++;
        }
    }
   
    // Initialize start as 0
    // which indicates the
    // first diagonal
    let start = 0;
   
    // Initialize end as
    // M + N - 2 which indicates
    // the last diagonal
    let end = M + N - 2;
   
    // Number of elements in
    // the current diagonal
    let no_of_elemnts = 1;
   
    // Maximum possible number
    // of elements in a diagonal
    // can be minimum of (number of
    // rows and number of columns)
    let max_elements = Math.min(M, N);
   
    while (start < end)
    {
   
        // The frequency of number
        // which occurs for the
        // maximum number of times
        // in the two selected
        // diagonals
        let X = Number.MIN_VALUE;
   
        for(let i = 0; i <= 10000; i++)
        {
            X = Math.max(X,
                frequency_diagonal[start][i] +
                frequency_diagonal[end][i]);
        }
   
        answer = answer + (2 * (no_of_elemnts)) - X;
   
        // Increment start
        start++;
           
        // Decrement end
        end--;
   
        // Increment current number
        // of elements until it reaches
        // the maximum possible value
        if (no_of_elemnts < max_elements)
            no_of_elemnts++;
    }
   
    // return the final answer
    return answer;
}
 
// Driver Code
// Number of rows
    let M = 3;
   
    // Number of columns
    let N = 7;
   
    let mat = [[ 1, 2, 3, 4, 5, 6, 7 ],
                    [ 2, 2, 3, 3, 4, 3, 2 ],
                    [ 1, 2, 3, 2, 5, 6, 4 ]];
   
    document.write(MinReplacements(M, N, mat) + "<br>");
 
// This code is contributed by avanitrachhadiya2155
</script>
Output: 
10

 

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

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