Write a Java program that takes a 2D bitmap (represented as a 1D array of integers), and reverses it about its vertical axis.

Question

idispatch · Accepted Answer

In Python:

array1d = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]

#-------------------------------------------------
def flipV(arr, width):
    rows = len(arr)//width
    for i in range(rows): # [0..rows)
        start = i * width       # index of first element in row
        end = start + width - 1 # index of last element in row
        while start < end:
            arr[start],arr[end] = arr[end],arr[start]
            start += 1
            end -= 1
#-------------------------------------------------
            
def print2D(arr, width):
    rows = len(arr)//width
    for i in range(rows): # [0..rows)
        rowStart = i * width # start of row(i)
        for j in range(width): # [0..width)
            print(arr[rowStart + j], " ", end='')
        print()

print(array1d) # original
print2D(array1d, 4)

flipV(array1d, 4)

print(array1d) # flipped
print2D(array1d, 4)

idispatch · Answer

Result:
>>> 
Input
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
0  1  2  3  
4  5  6  7  
8  9  10  11  
12  13  14  15  

Output
[3, 2, 1, 0, 7, 6, 5, 4, 11, 10, 9, 8, 15, 14, 13, 12]
3  2  1  0  
7  6  5  4  
11  10  9  8  
15  14  13  12

Anonymous · Answer

I asked the interviewer whether I'm also provided with a width, so that I know how to interpret the data in the 1D bitmap, and he said yes.  From a high level, you need a doubly nested for loop: the outer one iterates over the rows, and the inner one iterates over the elements, reversing them as you go along.

Matt · Answer

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