Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 20 | Hard
20.11 Imagine you have a square matrix, where each cell is filled with either black or white.
Design an algorithm to find the maximum subsquare such that all four borders are
filled with black pixels.
SOLUTION
Assumption: Square is of size NxN.
This algorithm does the following:
1. Iterate through every (full) column from left to right.
2. At each (full) column (call this currentColumn), look at the subcolumns (from biggest
to smallest).
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3. At each subcolumn, see if you can form a square with the subcolumn as the left side. If
so, update currentMaxSize and go to the next (full) column.
4. If N - currentColumn currentMaxSize) { // See step 4 above
15 for (int row = 0; row < matrix.length; row++){
16 // starting from the biggest
17 int size = N - Math.max(row, col);
18 while (size > currentMaxSize){
19 if (isSquare(matrix, row, col, size)){
20 currentMaxSize = size;
21 sq = new Subsquare(row, col, size);
22 break; // go to next (full) column
2 9 3
Cracking the Coding Interview | Additional Review Problems