Problem

Problem_Link

My Attempt

Brainstorming

X-Matrix All the elements in the diagonals of the matrix are non-zero. All other elements are 0.

So, it need to have 0 in all positions.

num, 0,0,0,num

0,num,0,num,0

0,0,num,0,0

0,num,0,num,0

num, 0,0,0,num

n == grid.length == grid[i].length
3 <= n <= 100
0 <= grid[i][j] <= 105

should count nums. it less than 0's

5/2=2 2/2=1

10/2=5

1000000001

0100000010

0010000100

0001001000

0000110000

0001001000

0010000100

0100000010

1000000001

loop until length/2 for nums. if(i==length/2) x=-1

i+x to check pos.

Result code

class Solution {
    public boolean checkXMatrix(int[][] grid) {

        int len = grid.length;

        for (int i = 0; i < (grid.length / 2); ++i) {
            if (!chkArray(grid[i], i + 1)) return false;
            if (!chkArray(grid[len - 1 - i], i + 1)) return false;
        }

        return true;
    }


    public static boolean chkArray(int[] grid, int count) {

        for (int i = 0; i < grid.length; ++i) {
            if (grid[i] != 0) {

                if (i != (count - 1) || i != (grid.length - count)) {
                    return false;
                }


            }

        }
        return true;

    }

}

Wrong Answer

Study

chkArray method was wrong. logic was fine. but the code is wrong. I needed 20mins more+IDE to fix it. need to practice hard code.

Solution2 is using Recursion. For me, this is not easy to understand.. need to study.

Solution (time, space)

O(n/2), O(1)

class Solution {
    public boolean checkXMatrix(int[][] grid) {
        for (int i = 0; i < (grid.length / 2); ++i) {
            if (!chkArray(grid[i], i + 1)) return false;
            if (!chkArray(grid[grid.length - 1 - i], i + 1)) return false;
        }
        return true;
    }

    public static boolean chkArray(int[] grid, int count) {
        for (int i = 0; i < grid.length; ++i) {
            if (i != (count - 1) && i != (grid.length - count)) {
                if (grid[i] != 0) return false;
            } else {
                if (grid[i] == 0) return false;
            }
        }
        return true;
    }
}

Runtime: 1 ms, faster than 99.85% of Java online submissions for Check if Matrix Is X-Matrix. Memory Usage: 53.9 MB, less than 83.86% of Java online submissions for Check if Matrix Is X-Matrix.

Solution2 (time, space)

O(n/2), O(1)

class Solution {
    public boolean checkXMatrix(int[][] grid) {
        return checkRect(grid, 0, grid.length - 1);
    }

    boolean checkRect(int[][] grid, int first, int last) {
        if (grid[first][first] == 0 ||
                grid[first][last] == 0 ||
                grid[last][first] == 0 ||
                grid[last][last] == 0) return false;
        for (int i = first + 1; i < last; i++) {
            if (grid[first][i] != 0 ||
                    grid[last][i] != 0 ||
                    grid[i][first] != 0 ||
                    grid[i][last] != 0) return false;
        }
        if (first + 1 >= last) return true;
        return checkRect(grid, first + 1, last - 1);
    }
}