leetcode nummatrix

import java.util.ArrayList;

public class  {
  
      Given a 2D matrix matrix, find the sum of the elements inside the rectangle defined by its upper left corner (row1, col1) and lower right corner (row2, col2).

      Range Sum Query 2D
      The above rectangle (with the red border) is defined by (row1, col1) = (2, 1) and (row2, col2) = (4, 3), which contains sum = 8.

      Example:
      Given matrix = [
        [3, 0, 1, 4, 2],
        [5, 6, 3, 2, 1],
        [1, 2, 0, 1, 5],
        [4, 1, 0, 1, 7],
        [1, 0, 3, 0, 5]
      ]

      sumRegion(2, 1, 4, 3) -> 8
      sumRegion(1, 1, 2, 2) -> 11
      sumRegion(1, 2, 2, 4) -> 12
   */
  int[][] matrix;
  public (int[][] matrix) {
    int n = matrix.length;
    int m = matrix[0].length;

    if (matrix == null || matrix.length == 0){
      return ;
    }
    this.matrix = new int[n][m];
    for(int i=0; i<n; i++){
      for (int j=0; j<m; j++){
        if (i==0 && j==0){
          this.matrix[i][j] = matrix[i][j];
        }
        else if (i == 0){
          this.matrix[i][j] = matrix[i][j] + this.matrix[i][j-1];
        }
        else if (j == 0){
          this.matrix[i][j] = matrix[i][j] + this.matrix[i-1][j];
        }
        else{
          this.matrix[i][j] = matrix[i][j] + this.matrix[i-1][j] + this.matrix[i][j-1] - this.matrix[i-1][j-1];
        }
      }
    }

  }
  public int sumRegion(int row1, int col1, int row2, int col2) {
    if (col1 == 0 && row1 == 0){
      return matrix[row2][col2];
    }
    else if (col1 == 0){
      return matrix[row2][col2] - matrix[row1-1][col2];
    }
    else if (row1 == 0){
      return matrix[row2][col2] - matrix[row2][col1-1];
    }
    else
      return matrix[row2][col2] - matrix[row2][col1-1] - matrix[row1-1][col2] + matrix[row1-1][col1-1];
  }
}