1329. Sort the Matrix Diagonally 

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class Solution {
    public int[][] diagonalSort(int[][] mat) {
        int m = mat.length, n = mat[0].length;

        List<Integer> numList = new ArrayList<>();

        // right to left
        for (int j = n - 1; j >= 0; j--) {
            numList.clear();

            for (int x = 0, y = j; x < m && y < n; x++, y++) {
                numList.add(mat[x][y]);
            }
            Collections.sort(numList);
            for (int x = 0, y = j, k = 0; x < m && y < n; x++, y++, k++) {
                mat[x][y] = numList.get(k);
            }
        }

        // top to bottom
        for (int i = 1; i < m; i++) {
            numList.clear();

            for (int x = i, y = 0; x < m && y < n; x++, y++) {
                numList.add(mat[x][y]);
            }
            Collections.sort(numList);
            for (int x = i, y = 0, k = 0; x < m && y < n; x++, y++, k++) {
                mat[x][y] = numList.get(k);
            }
        }

        return mat;
    }
}

References

1329. Sort the Matrix Diagonally