Range Sum Query 2D - Immutable

private int[][] sum;
public NumMatrix(int[][] matrix) {
    int row = matrix.length;
    if(row == 0) return;
    int col = matrix[0].length;
    sum = new int[row][col];
    sum[0][0] = matrix[0][0];
    for(int i=1;i<row;i++) sum[i][0] = sum[i-1][0]+matrix[i][0];
    for(int j=1;j<col;j++) sum[0][j] = sum[0][j-1]+matrix[0][j];

    for(int i=1;i<row;i++){
        for(int j = 1;j<col;j++){
            sum[i][j] = sum[i][j-1]+sum[i-1][j]-sum[i-1][j-1]+matrix[i][j];
        }
    }
} 

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