Sort the Matrix Diagonally

Given a m * n matrix mat of integers, sort it diagonally in ascending order from the top-left to the bottom-right then return the sorted array.

Example 1:

Input: mat = [[3,3,1,1],[2,2,1,2],[1,1,1,2]]
Output: [[1,1,1,1],[1,2,2,2],[1,2,3,3]]

Constraints:

m == mat.length
n == mat[i].length
1 <= m, n <= 100
1 <= mat[i][j] <= 100

Solution:

class Solution {
    public int[][] diagonalSort(int[][] mat) {
        int m = mat.length;
        int n = mat[0].length;
        int k = m + n - 1;
        for (int i = m - 1; i >= 0; i --) {
            int[] arr = new int[Math.min(n, m - i)];
            int x = i, y = 0, j = 0;
            while (x < m && y < n) {
                arr[j ++] = mat[x ++][y ++];
            }
            Arrays.sort(arr);
            x = i;
            y = 0;
            j = 0;
            while (x < m && y < n) {
                mat[x ++][y ++] = arr[j ++];
            }
        }
        
        for (int j = 1; j < n; j ++) {
            int[] arr = new int[Math.min(m, n - j)];
            int x = 0, y = j, i = 0;
            while (x < m && y < n) {
                arr[i ++] = mat[x ++][y ++];
            }
            Arrays.sort(arr);
            x = 0;
            y = j;
            i = 0;
            while (x < m && y < n) {
                mat[x ++][y ++] = arr[i ++];
            }
        }
        return mat;
    }
}