At the first glance, this seems very easy. However, I couldn't figure out why we need to first mark the first row and first column if there is any 0 elements, check the rest of the matrix and set those rows and columns to zeros, then set first row or first column to zero if they are marked. Now I know.
Consider a matrix like this:
In the end, we want a matrix like this:
If we set zeros from the first row, after the first set, it will be like this:
In the end, it will be:
A disaster... :O
So, the correct way is to mark the first row and first column if there is any 0.
Check the rest of the matrix, if matrix[i][j] = 0, set matrix[i][0] = 0 and matrix[0][j] = 0.
Set row i and column j to zeros if matrix[i][0] = 0 or matrix[0][j] = 0.
Set first row and first column to zeros if they are marked.
public void setZeroes(int[][] matrix) { if (matrix == null) throw new NullPointerException("Null matrix!"); if (matrix.length == 0) return; boolean firstRow = false; boolean firstCol = false; for (int i = 0; i < matrix.length; i++) { if (matrix[i][0] == 0) { firstCol = true; break; } } for (int j = 0; j < matrix[0].length; j++) { if (matrix[0][j] == 0) { firstRow = true; break; } } for (int i = 1; i < matrix.length; i++) { for (int j = 1; j < matrix[0].length; j++) { if (matrix[i][j] == 0) { matrix[i][0] = 0; matrix[0][j] = 0; } } } for (int i = 1; i < matrix.length; i++) { for (int j = 1; j < matrix[0].length; j++) { if (matrix[i][0] == 0 || matrix[0][j] == 0) matrix[i][j] = 0; } } if(firstRow == true) { for (int j = 0; j < matrix[0].length; j++) matrix[0][j] = 0; } if (firstCol == true) { for (int i = 0; i < matrix.length; i++) matrix[i][0] = 0; } }
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