2017-09-13 LeetCode_240 Search a 2D Matrix II

240. Search a 2D Matrix II

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:

• Integers in each row are sorted in ascending from left to right.
• Integers in each column are sorted in ascending from top to bottom.

For example,

Consider the following matrix:

[  [1,   4,  7, 11, 15],  [2,   5,  8, 12, 19],  [3,   6,  9, 16, 22],  [10, 13, 14, 17, 24],  [18, 21, 23, 26, 30]]

Given target = 5, return true.

Given target = 20, return false.

solution:

class Solution {

2

public:

3

    bool divides(vector<vector<int> >& matrix, int target, int x0, int y0, int x1, int y1) {

4

        if (x0 > x1 || y0 > y1) return false;

5

        int x = (x0+x1)/2, y = (y0+y1)/2;

6

        if (matrix[x][y] == target) return true;

7

        if (matrix[x][y] < target)

8

            return divides(matrix, target, x+1, y+1, x1, y1) ||

9

                divides(matrix, target, x+1, y0, x1, y) ||

10

                divides(matrix, target, x0, y+1, x, y1);

11

        else

12

            return divides(matrix, target, x0, y0, x-1, y-1) ||

13

                divides(matrix, target, x0, y, x-1, y1) ||

14

                divides(matrix, target, x, y0, x1, y-1);

15

    }

16

    bool searchMatrix(vector<vector<int> >& matrix, int target) {

17

        if (matrix.size() == 0 || matrix[0].size() == 0) return false;

18

        return divides(matrix, target, 0, 0, matrix.size()-1, matrix[0].size()-1);

19

    }

20

};