typescript
function diagonalDifference(arr: number[][]): number {
  let left = 0, right = 0, row = 0;
  for (
    let i = 0, j = arr.length - 1;
    i < arr.length && j >= 0; 
    i++, j--
  ) {
    left += arr[row][i];
    right += arr[row][j];
    row++;
  }
  return Math.abs(left - right);
}