73. Set Matrix Zeroes

题目：

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

链接： http://leetcode.com/problems/set-matrix-zeroes/

题解：

先判断矩阵第一行和第一列是否需要清零，然后再扫描矩阵把需要清零的行和列标注在第一行和第一列里， 最后进行清零操作。

Time Complexity - O(m x n)， Space Complexity - O(1)。

public class Solution {
    public void setZeroes(int[][] matrix) {
        if(matrix == null || matrix[0].length == 0)
            return;
        int m = matrix.length, n = matrix[0].length;    
        Boolean firstRowZero = false, firstColZero = false;
        
        for(int i = 0; i < m; i ++){
            if(matrix[i][0] == 0){
                firstColZero = true;
                break;
            }
        }
        
        for(int j = 0; j < n; j ++){
            if(matrix[0][j] == 0){
                firstRowZero = true;
                break;
            }
        }
        
        for(int i = 1; i < m; i ++){
            for(int j = 1; j < n; j++){
                if(matrix[i][j] == 0){
                    matrix[i][0] = 0;
                    matrix[0][j] = 0;
                }
            }
        }
        
        for(int i = 1; i < m; i ++){
            for(int j = 1; j < n; j ++){
                if(matrix[i][0] == 0 || matrix[0][j] == 0)
                    matrix[i][j] = 0;
            }
        }
        
        if(firstRowZero == true)
            for(int j = 0; j < n; j ++)
                matrix[0][j] = 0;
        
        if(firstColZero == true)
            for(int i = 0; i < m; i ++)
                matrix[i][0] = 0;
    }
}

Update:

public class Solution {
    public void setZeroes(int[][] matrix) {
        if(matrix == null || matrix.length == 0)
            return;
        boolean isZeroInFirstRow = false, isZeroInFirstCol = false;
        
        for(int i = 0; i < matrix.length; i++)
            if(matrix[i][0] == 0) {
                isZeroInFirstCol = true;
                break;
            }
        
        for(int j = 0; j < matrix[0].length; j++)
            if(matrix[0][j] == 0) {
                isZeroInFirstRow = 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(isZeroInFirstCol)
            for(int i = 0; i < matrix.length; i++)
                matrix[i][0] = 0;
        
        if(isZeroInFirstRow) {
            for(int j = 0; j < matrix[0].length; j++)
                matrix[0][j] = 0;
        }
    }
}

二刷：

题目不难但是打字比较费力。

Java:

Time Complexity - O(mn)， Space Complexity - O(1)

public class Solution {
    public void setZeroes(int[][] matrix) {
        if (matrix == null || matrix.length == 0) {
            return;
        }
        boolean hasZeroFirstRow = false, hasZeroFirstCol = false;
        for (int i = 0; i < matrix.length; i++) {
            if (matrix[i][0] == 0) {
                hasZeroFirstCol = true;
                break;
            }
        }
        for (int j = 0; j < matrix[0].length; j++) {
            if (matrix[0][j] == 0) {
                hasZeroFirstRow = 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 (hasZeroFirstRow) {
            for (int j = 0; j < matrix[0].length; j++) {
                matrix[0][j] = 0;
            }
        }
        if (hasZeroFirstCol) {
            for (int i = 0; i < matrix.length; i++) {
                matrix[i][0] = 0;
            }
        }
    }
}

三刷:

Java:

public class Solution {
    public void setZeroes(int[][] matrix) {
        if (matrix == null || matrix.length == 0) return;
        boolean hasZeroFirstRow = false, hasZeroFirstCol = false;
        for (int i = 0; i < matrix.length; i++) {
            if (matrix[i][0] == 0) {
                hasZeroFirstCol = true;
                break;
            }
        }
        
        for (int j = 0; j < matrix[0].length; j++) {
            if (matrix[0][j] == 0) {
                hasZeroFirstRow = 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 (hasZeroFirstRow) {
            for (int j = 0; j < matrix[0].length; j++) matrix[0][j] = 0;
        }
        if (hasZeroFirstCol) {
            for (int i = 0; i < matrix.length; i++) matrix[i][0] = 0;
        }
    }
}

测试: