LeetCode-Range Sum Query 2D

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).

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
update(3, 2, 2)
sumRegion(2, 1, 4, 3) -> 10

Note:

1. The matrix is only modifiable by the update function.
2. You may assume the number of calls to update and sumRegion function is distributed evenly.
3. You may assume that row1 ≤ row2 and col1 ≤ col2

Analysis:2D BIT Practice.

NOTE: CAREFULL about the gap between BIT's index and normal matrix index. We need be consistent when define functions.

Solution:

public class NumMatrix {
    int[][] sums;
    int[][] nums;
    int rows, cols;

    public NumMatrix(int[][] matrix) {
        if (matrix.length==0 || matrix[0].length==0) return;

        rows = matrix.length;
        cols = matrix[0].length;
        sums = new int[rows+1][cols+1];
        nums = new int[rows][cols];
        for (int i=0;i<rows;i++)
            for (int j=0;j<cols;j++){
                update(i,j,matrix[i][j]);
            }
    }

    public void update(int row, int col, int val) {
        if (rows==0 || cols==0) return;
        
        int delta = val - nums[row][col];
        nums[row][col] = val;
        for (int x=row+1;x<=rows;x = x + (x & -x))
            for (int y=col+1;y<=cols;y = y + (y & -y)){
                sums[x][y] += delta;
            }
    }

    public int sumRegion(int row1, int col1, int row2, int col2) {
        if (rows==0 || cols == 0) return 0;
        
        return sum(row2,col2) - sum(row2,col1-1) - sum(row1-1,col2) + sum(row1-1,col1-1);
    }

    // @row and @col still stand for the index in @nums not @sums, so we always perform index transformation in loops.
    public int sum(int row, int col){
        int sum = 0;
        for (int x = row+1; x>0; x -= (x & -x))
            for (int y = col+1; y>0; y -= (y & -y)){
                sum += sums[x][y];
            }
        return sum;
    }
}


// Your NumMatrix object will be instantiated and called as such:
// NumMatrix numMatrix = new NumMatrix(matrix);
// numMatrix.sumRegion(0, 1, 2, 3);
// numMatrix.update(1, 1, 10);
// numMatrix.sumRegion(1, 2, 3, 4);