Horizontally Flip a Binary Matrix

Difficulty Level : Easy
Last Updated : 21 May, 2021

Given a binary matrix. The task is to flip the matrix horizontally(find the image of the matrix), then invert it.

Note:

To flip a matrix horizontally means that reversing each row of the matrix. For example, flipping [1, 1, 0, 0] horizontally results in [0, 0, 1, 1].
To invert a matrix means that replacing each 0 by 1 and vice-versa. For example, inverting [0, 0, 1] results in [1, 1, 0].

Examples:

Input: mat[][] = [[1, 1, 0], 
                  [1, 0, 1], 
                  [0, 0, 0]]
Output: [[1, 0, 0], 
         [0, 1, 0], 
         [1, 1, 1]]
Explanation: 
First reverse each row: [[0, 1, 1], [1, 0, 1], [0, 0, 0]]
Then, invert the image: [[1, 0, 0], [0, 1, 0], [1, 1, 1]]

Input: mat[][] = [[1, 1, 0, 0], 
                  [1, 0, 0, 1], 
                  [0, 1, 1, 1], 
                  [1, 0, 1, 0]]
Output: [[1, 1, 0, 0], 
         [0, 1, 1, 0], 
         [0, 0, 0, 1], 
         [1, 0, 1, 0]]
Explanation: 
First reverse each row: 
[[0, 0, 1, 1], [1, 0, 0, 1], [1, 1, 1, 0], [0, 1, 0, 1]].
Then invert the image:
[[1, 1, 0, 0], [0, 1, 1, 0], [0, 0, 0, 1], [1, 0, 1, 0]]

Approach: We can do this in-place. On observing carefully, it can be deduced that in each row in the final matrix, the i-th value from the left is equal to the inverse of the i-th value from the right of the input binary matrix.

Thus, we use (Column + 1) / 2 (with floor division) to iterate over all indexes $i$ in the first half of the row, including the centre and updating the answer accordingly.

Below is the implementation of above approach:

Python3

# Python3 implementation of above approach
# Function to return final Image
 
def flipped_Invert_Image(mat):
 
    for row in mat:
        for i in range((len(row) + 1) // 2):
 
            row[i] = row[len(row) - 1 - i] ^ 1
            row[len(row) - 1 - i] = row[i] ^ 1
 
    # return Flipped and Inverted image
    return mat
 
# Driver code
mat = [[1, 1, 0, 0], [1, 0, 0, 1], [0, 1, 1, 1], [1, 0, 1, 0]]
 
print(flipped_Invert_Image(mat))

Output:

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

Time Complexity: O(N*M), where N is the number of rows and M is the number of columns in the given binary matrix.

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