Sort the Matrix Diagonally

A matrix diagonal is a diagonal line of cells starting from some cell in either the topmost row or leftmost column and going in the bottom-right direction until reaching the matrix's end. For example, the matrix diagonal starting from mat[2][0], where mat is a 6 x 3 matrix, includes cells mat[2][0], mat[3][1], and mat[4][2].

Given an m x n matrix mat of integers, sort each matrix diagonal in ascending order and return the resulting matrix.

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

S0lution:

class Solution {
    public int[][] diagonalSort(int[][] mat) {
        for(int i=0;i<mat.length;i++)
        {
            sortdiagnol(mat,i,0);
            
        }
        for(int j=0;j<mat[0].length;j++)
        {
            sortdiagnol(mat,0,j);
        }
        return mat;
    }
    
    public void sortdiagnol(int[][] mat, int m, int n)
    {
        int row = mat.length;
        int col = mat[0].length;
         ArrayList<Integer> list = new ArrayList<>();
        for(int i=m,j=n; i<row && j<col; i++,j++)
        {
           list.add(mat[i][j]);
        }
        Collections.sort(list);
        for(int k:list)
        {
            mat[m++][n++]=k;
        }
    }
}