In matrix, if a zero is found, make its row and column to all zero

Question

I am making an ArrayList, keeping note of all the indices where zero has occurred. Later on, I am taking out the indices from ArrayList and making the corresponding row and column to zero.

Please suggest improvement and the better ways to do it in place.

     public void setZeroes(int[][] matrix) {
        if(matrix.length == 0 ) {
            return;
        }
        if(matrix.length == 1) {
            boolean flag = false;
            for(int j =0; j<matrix[0].length; j++) {
                if(matrix[0][j] == 0){
                    flag = true;
                    break;
                }
            }
            if(flag) {
                for(int j =0; j<matrix[0].length; j++) {
                    matrix[0][j] = 0;
                    }
                }
            return;
        }
        ArrayList<MyHelperClass> zeroList = new ArrayList<>();
        int m = matrix.length;
        int n = matrix[0].length;
        for(int i =0; i<m; i++) {
            for(int j =0; j<n; j++) {
                if(matrix[i][j] == 0) {
                    MyHelperClass mh = new MyHelperClass(i, j);
                    zeroList.add(mh);
                }
            }
        }

        Iterator ie = zeroList.iterator();
        while(ie.hasNext()) {
            MyHelperClass mh2 = (MyHelperClass) ie.next();
            int x = mh2.x;
            int y = mh2.y;
            for(int j =0; j<n; j++) {
                matrix[x][j] = 0;
                //matrix[i][y] = 0;
            }
            for(int i =0; i<m; i++) {
                matrix[i][y] = 0;
                //matrix[i][y] = 0;
            }
        }


    }

 private static class MyHelperClass {
     int x;
     int y;
     public MyHelperClass(int x, int y){
         this.x = x;
         this.y = y;
     }
 }

Answer 1

If I get you question right you want to transform a matrix from

1 1 1 1 1
1 1 0 1 1
1 1 1 1 1
1 1 1 1 1

to

1 1 0 1 1
0 0 0 0 0
1 1 0 1 1
1 1 0 1 1

forming a "cross" of zeros for each zero in the original matrix. Your code is pretty hard to read and understand.

Here is how I'd do it:

1. Iterate over the matrix looking for 0
2. Remember the x and y coordinates of a 0 in Sets
3. Iterate over both Sets for x-coordinates and y-coordinates
4. Setting all values on column x to 0
5. Setting all values on row y to 0

Here is some code

    private static void replaceInRowAndColumn(int search, int replace, int[][] matrix) {
        RowAndColumnIndexesSets allRowAndColumnIndexes = findAllRowAndColumnIndexesOf(search, matrix);
        for (Integer x : allRowAndColumnIndexes.rows) {
            for (int y = 0; y < matrix[x].length; y++) {
                matrix[x][y] = replace;
            }
        }
        for (int x = 0; x < matrix.length; x++) {
            for (Integer y : allRowAndColumnIndexes.columns) {
                matrix[x][y] = replace;
            }
        }
    }

    private static RowAndColumnIndexesSets findAllRowAndColumnIndexesOf(int search, int[][] matrix) {
        Set<Integer> rowIndexes = new HashSet<Integer>();
        Set<Integer> columnIndexes = new HashSet<Integer>();
        for (int row = 0; row < matrix.length; row++) {
            Set<Integer> allIndexesInCurrentRow = findAllIndexesOf(search, matrix[row]);
            if (!allIndexesInCurrentRow.isEmpty()) {
                columnIndexes.addAll(allIndexesInCurrentRow);
                rowIndexes.add(row);
            }
        }
        return new RowAndColumnIndexesSets(rowIndexes, columnIndexes);
    }

    private static Set<Integer> findAllIndexesOf(int search, int[] row) {
        Set<Integer> indexes = new HashSet<Integer>();
        for (int i = 0; i < row.length; i++) {
            if (row[i] == search) {
                indexes.add(i);
            }
        }
        return indexes;
    }

    private static class RowAndColumnIndexesSets {
        private final Set<Integer> rows;
        private final Set<Integer> columns;

        public RowAndColumnIndexesSets(Set<Integer> rows, Set<Integer> columns) {
            this.rows = rows;
            this.columns = columns;
        }

        public Set<Integer> getRows() {
            return rows;
        }

        public Set<Integer> getColumns() {
            return columns;
        }
    }

Keep in mind that you should try to split you code up into smaller logical (reusable) methods and give them a meaningful name. Also it's not necessary for your problem to remember the coordinate tuple (x,y) of zeros in the matrix. A Set for column indexes and a Set for row indexes is enough. Sets have the advantage that they do not contain duplicate values. If you have a zero at (1,3) and another zero at (2,3) you have to fill zeros in column 3 only once.