• 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.

    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?

    Time O(nm) space O(N+M)
    public class Solution {
        public void setZeroes(int[][] matrix) {
            int col = matrix[0].length;
            int row = matrix.length;
            HashSet<Integer> row0 = new HashSet<>();
            HashSet<Integer> col0 = new HashSet<>();
            for(int i = 0 ; i < row ; i++){
                for(int j = 0 ; j < col; j++){
                    if(matrix[i][j] == 0){
                        row0.add(i);
                        col0.add(j);
                    }
                }
            }
            for(int i : row0){
                for(int j = 0 ; j < col; j++){
                    matrix[i][j] = 0;
                }
            }
            
            for(int i : col0){
                for(int j = 0 ; j < row; j++){
                    matrix[j][i] = 0;
                }
            }
        }
    }

    法二 : no space

       用每行的第一个 和每列的第一来存0,注意清零时不能用第一列和第一行 否则会相互影响。

        //No extra space  只要有0,用该row或者该col的第一个值存0
        public void setZeroes(int[][] matrix) {
            int col = matrix[0].length;
            int row = matrix.length;
            int col0 = 1;
            for(int i = 0 ; i < row ; i++){
                if (matrix[i][0] == 0) col0 = 0;
                for(int j = 1 ; j < col; j++){
                    if(matrix[i][j] == 0){
                        matrix[i][0] = 0;
                        matrix[0][j] = 0;   
                    }
                }
            }
            for(int i = row -1 ; i >= 0 ; i--){
                for(int j = col - 1 ; j >= 1; j--){
                    if(matrix[i][0] == 0 ||  matrix[0][j] == 0)
                        matrix[i][j] = 0;
                }
            }
            
            if(col0 == 0){
                for(int i = 0; i < row ; i++){
                    matrix[i][0] = 0;
                }
            }
        }
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  • 原文地址:https://www.cnblogs.com/joannacode/p/6104171.html
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