308. Range Sum Query 2D - Mutable

class NumMatrix {

    Fenwick[] bit;
    public NumMatrix(int[][] matrix) {
        int m = matrix.length;
        bit = new Fenwick[m];
        for(int i = 0; i < m; i++)
        {
            bit[i] = new Fenwick(matrix[i]);
            System.out.println(Arrays.toString(bit[i].bit));
        }

    }

    public void update(int row, int col, int val) {
        bit[row].update(col + 1, val);
    }

    public int sumRegion(int row1, int col1, int row2, int col2) {
        int sum = 0;
        for(int i = row1; i <= row2; i++)
        {
            sum += bit[i].get(col2 + 1) - bit[i].get(col1);
        }
        return sum;
    }
}

class Fenwick
{
    int[] bit;

    public Fenwick(int[] ary)
    {
        int n = ary.length, len = 1;
        while(len <= n) len <<= 1;
        bit = new int[len];
        for(int i = 0; i < n; i++) update(i + 1, ary[i]);
    }

    int get(int pos)
    {
        int res = 0;
        while(pos > 0)
        {
            res += bit[pos];
            pos -= pos & -pos;
        }
        return res;
    }

    void update(int pos, int val)
    {
        int cur = get(pos) - get(pos - 1), diff = -cur + val;
        int n = bit.length;
        while(pos < n)
        {
            bit[pos] += diff;
            pos += pos & -pos;
        }
    }
}

/**
 * Your NumMatrix object will be instantiated and called as such:
 * NumMatrix obj = new NumMatrix(matrix);
 * obj.update(row,col,val);
 * int param_2 = obj.sumRegion(row1,col1,row2,col2);
 */