Binary Matrix after flipping submatrices in given range for Q queries

Binary Matrix after flipping submatrices in given range for Q queries

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Given a binary matrix arr[][] of dimensions M x N and Q queries of the form (x1, y1, x2, y2), where (x1, y1) and (x2, y2) denotes the top-left and bottom-right indices of the submatrix required to be flipped(convert 0s to 1s and vice versa) respectively. The task is to print the final matrix obtained after performing given Q queries

Examples:

Input:_ arr[][] = {{0, 1, 0}, {1, 1, 0}}, queries[][] = {{1, 1, 2, 3}}_

Output:_ [[1, 0, 1], [0, 0, 1]]_

Explanation:

_The submatrix to be flipped is equal to {{0, 1, 0}, {1, 1, 0}} _

The flipped matrix is {{1, 0, 1}, {0, 0, 1}}.

Input:_ arr[][] = {{0, 1, 0}, {1, 1, 0}}, queries[][] = {{1, 1, 2, 3}, {1, 1, 1, 1], {1, 2, 2, 3}}_

Output:_ [[0, 1, 0], [0, 1, 0]]_

Explanation:

Query 1:

_Submatrix to be flipped = [[0, 1, 0], [1, 1, 0]] _

_Flipped submatrix is [[1, 0, 1], [0, 0, 1]]. _

Therefore, the modified matrix is [[1, 0, 1], [0, 0, 1]].

_Query 2: _

_Submatrix to be flipped = [[1]] _

_Flipped submatrix is [[0]] _

Therefore, matrix is [[0, 0, 1], [0, 0, 1]].

Query 3:

_Submatrix to be flipped = [[0, 1], [0, 1]] _

_Flipped submatrix is [[1, 0], [1, 0]] _

Therefore, modified matrix is [[0, 1, 0], [0, 1, 0]].

Naive Approach: The simplest approach to solve the problem for each query is to iterate over the given submatrices and for every element, check if it is 0 or 1, and flip accordingly. After completing these operations for all the queries, print the final matrix obtained.

Below is the implementation of the above approach :

  • Python

## Python Program to implement

## the above approach

## Function to flip a submatrices

**def** manipulation(matrix, q):

## Boundaries of the submatrix

x1, y1, x2, y2 **=** q

## Iterate over the submatrix

**for** i **in** range``(x1``**-**``1``, x2):

**for** j **in** range``(y1``**-**``1``, y2):

## Check for 1 or 0

## and flip accordingly

**if** matrix[i][j]:

matrix[i][j] **=** 0

**else**``:

matrix[i][j] **=** 1

## Function to perform the queries

**def** queries_fxn(matrix, queries):

**for** q **in** queries:

manipulation(matrix, q)

## Driver Code

matrix **=** [[``0``, 1``, 0``], [``1``, 1``, 0``]]

queries **=** [[``1``, 1``, 2``, 3``], \

[``1``, 1``, 1``, 1``], \

[``1``, 2``, 2``, 3``]]

## Function call

queries_fxn(matrix, queries)

print``(matrix)

Output:

[[0, 1, 0], [0, 1, 0]]

Time complexity:_ O( N * M * Q)_

Auxiliary Space:_ O(1)_

Efficient Approach: The above approach can be optimized using Dynamic programming and Prefix Sum technique. Mark the boundaries of the submatrices involved in each query and then calculate prefix sum of the operations involved in the matrix and update the matrix accordingly. Follow the steps below to solve the problem:

  • Initialize a 2D state space table dp[][] to store the count of flips at respective indices of the matrix
  • For each query {x1, y1, x2, y2, K}, update the dp[][] matrix by the following operations:
  • dp[x1][y1] += 1
  • dp[x2 + 1][y1] -= 1
  • dp[x2 + 1][y2 + 1] += 1
  • dp[x1][y2 + 1] -= 1

Now, traverse over the dp[][] matrix and update dp[i][j] by calculating prefix sum of the rows and colums and diagonals by the following relation:

dp[i][j] = dp[i][j] + dp[i-1][j] + dp[i][j – 1] – dp[i – 1][j – 1]

If dp[i][j] is found to be odd, reduce mat[i – 1][j – 1] by 1.

Finally print the updated matrix mat[][] as the result.

Below is the implementation of the above approach :

  • Python

## Python program to implement

## the above approach

## Function to modify dp[][] array by

## generating prefix sum

**def** modifyDP(matrix, dp):

**for** j **in** range``(``1``, len``(matrix)``**+**``1``):

**for** k **in** range``(``1``, len``(matrix[``0``])``**+**``1``):

## Update the tabular data

dp[j][k] **=** dp[j][k] **+** dp[j``**-**``1``][k] \

**+** dp[j][k``**-**``1``]``**-**``dp[j``**-**``1``][k``**-**``1``]

## If the count of flips is even

**if** dp[j][k] **%** 2 !``**=** 0``:

matrix[j``**-**``1``][k``**-**``1``] **=** int``(matrix[j``**-**``1``][k``**-**``1``]) ^ 1

## Function to update dp[][] matrix

## for each query

**def** queries_fxn(matrix, queries, dp):

**for** q **in** queries:

x1, y1, x2, y2 **=** q

## Update the table

dp[x1][y1] **+=** 1

dp[x2 **+** 1``][y2 **+** 1``] **+=** 1

dp[x1][y2 **+** 1``] **-=** 1

dp[x2 **+** 1``][y1] **-=** 1

modifyDP(matrix, dp)

## Driver Code

matrix **=** [[``0``, 1``, 0``], [``1``, 1``, 0``]]

queries **=** [[``1``, 1``, 2``, 3``], \

[``1``, 1``, 1``, 1``], \

[``1``, 2``, 2``, 3``]]

## Initialize dp table

dp **=** [[``0 **for** i **in** range``(``len``(matrix[``0``])``**+**``2``)] \

**for** j **in** range``(``len``(matrix)``**+**``2``)]

queries_fxn(matrix, queries, dp)

print``(matrix)

Output:

[[0, 1, 0], [0, 1, 0]]

Time Complexity:_ O(N * M )_

Auxiliary Space:_ O(N * M)_

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.

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