LeetCode: Set Matrix Zeroes 解题报告

Set Matrix Zeroes
Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in place.

click to show follow up.

Follow up:
Did you use extra space?
A straight forward solution using O(mn) space is probably a bad idea.
A simple improvement uses O(m + n) space, but still not the best solution.
Could you devise a constant space solution?

SOLUTION 1:

题目要求O(1)的空间消耗。

1. 我们可以使用第一行，第一列来作为Flag，记录某一行，某一列是否应该被设置为0.

2. 因为第一行，第一列共用左上角的flag，所以我们需要另外找2个flag来定义第一行，第一列本身是否应该设置为0.

row1Zero， col1Zero

3. 先扫描首行，首列把首行首列的flag算出。

4. 扫描其他的矩阵，将第一行每一列的flag算出。

5. 设置矩阵中除了首行首列的cells.

6. 设置首行，设置首列。

 public class Solution {
     public void setZeroes(int[][] matrix) {
         if (matrix == null || matrix.length ==
             || matrix[].length == ) {
             return;
         }

         boolean row1Zero = false;
         boolean col1Zero = false;

         int rows = matrix.length;
         int cols = matrix[].length;

         // Determine if the first column should be Zero.
         for (int i = ; i < rows; i++) {
             if (matrix[i][] == ) {
                 col1Zero = true;
                 break;
             }
         }

         // Determine if the first row should be Zero.
         for (int i = ; i < cols; i++) {
             if (matrix[][i] == ) {
                 row1Zero = true;
                 break;
             }
         }

         // we use the first row and the first col as the flag to record the
         // cells whether or not set to 0.
         for (int i = ; i < rows; i++) {
             for (int j = ; j < cols; j++) {
                 // 注意了，这个矩阵是0和非0，并不是0和1.
                 if (matrix[i][j] == ) {
                     // set the flag in the first line and the first column
                     matrix[i][] = ;
                     matrix[][j] = ;
                 }
             }
         }

         // set the inner cells.
         // Be careful: i, j start from 1.
         for (int i = ; i < rows; i++) {
             for (int j = ; j < cols; j++) {
                 if (matrix[i][] ==
                    || matrix[][j] == ) {
                     matrix[i][j] = ;
                 }
             }
         }

         // set the first row.
         if (row1Zero) {
             for (int i = ; i < cols; i++) {
                 matrix[][i] = ;
             }
         }

         // set the first col.
         if (col1Zero) {
             for (int i = ; i < rows; i++) {
                 matrix[i][] = ;
             }
         }

         return;
     }
 }

2014.1230 redo:

把前面三个循环合并，会简单一点儿。

 public class Solution {
     public void setZeroes(int[][] matrix) {
         if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
             return;
         }

         boolean row1 = false;
         boolean col1 = false;

         int rows = matrix.length;
         int cols = matrix[0].length;

         // set flags.
         for (int i = 0; i < rows; i++) {
             for (int j = 0; j < cols; j++) {
                 if (matrix[i][j] != 0) {
                     continue;
                 }

                 // set the flag of a column and a row.
                 matrix[0][j] = 0;
                 matrix[i][0] = 0;

                 //  get flag of first row.
                 if (i == 0) {
                     row1 = true;
                 }

                 // get flag of first column.
                 if (j == 0) {
                     col1 = true;
                 }
             }
         }

         // set the matrix.
         for (int i = 1; i < rows; i++) {
             for (int j = 1; j < cols; j++) {
                 if (matrix[0][j] == 0 || matrix[i][0] == 0) {
                     matrix[i][j] = 0;
                 }
             }
         }

         // set first column
         if (col1) {
             for (int i = 0; i < rows; i++) {
                 // bug 1: can't use matrix[i][j]
                 matrix[i][0] = 0;
             }
         }

         // set first row.
         if (row1) {
             for (int i = 0; i < cols; i++) {
                 matrix[0][i] = 0;
             }
         }
     }
 }

GitHub Code:

SetZeroes.java