[leetcode][73] Set Matrix Zeroes

Posted 2021-01-07 ekoeko

73. 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.

Example 1:

Input: 
[
  [1,1,1],
  [1,0,1],
  [1,1,1]
]
Output: 
[
  [1,0,1],
  [0,0,0],
  [1,0,1]
]

Example 2:

Input: 
[
  [0,1,2,0],
  [3,4,5,2],
  [1,3,1,5]
]
Output: 
[
  [0,0,0,0],
  [0,4,5,0],
  [0,3,1,0]
]

Follow up:

• 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?

解析

把矩阵中含有0的那一行和列全部置0.

参考答案

自己写的：

class Solution {
    public void setZeroes(int[][] matrix) {
        Set<Integer> rows = new HashSet<>();
        Set<Integer> cols = new HashSet<>();
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix[i].length; j++) {
                if (matrix[i][j] == 0) {
                    rows.add(i);
                    cols.add(j);
                }
            }
        }
        for (Integer i : rows) {
            for (int j = 0; j < matrix[i].length; j++) {
                matrix[i][j] = 0;
            }
        }
        for (Integer i : cols) {
            for (int j = 0; j < matrix.length; j++) {
                matrix[j][i] = 0;
            }
        }
    }
}

一开始没注意题目的意思，这个是一般的解法，题目的意思是不要用额外的空间。

别人写的：

public class Solution {
    public void setZeroes(int[][] matrix) {
        boolean fr = false,fc = false;
        for(int i = 0; i < matrix.length; i++) {
            for(int j = 0; j < matrix[0].length; j++) {
                if(matrix[i][j] == 0) {
                    if(i == 0) fr = true;
                    if(j == 0) fc = true;
                    matrix[0][j] = 0;
                    matrix[i][0] = 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(fr) {
            for(int j = 0; j < matrix[0].length; j++) {
                matrix[0][j] = 0;
            }
        }
        if(fc) {
            for(int i = 0; i < matrix.length; i++) {
                matrix[i][0] = 0;
            }
        }
    }
}

把第0行和第0列留出来记录那些行和哪些列需要置0，用fr和fc来记录第0行和第0列是否需要置零。