二维树状数组

C[x][y] = ∑ a[i][j], 其中，
x-lowbit(x) + 1 <= i <= x,
y-lowbit(y) + 1 <= j <= y.

二维树状数组的更新和求和函数：

int lowbit(int x)
{
    return x&(-x);
}

void update(int x,int y,int val) 
{
    for(int i=x; i<=N; i+=lowbit(i))
    {
        for(int j=y; j<=N; j+=lowbit(j))
        {
            p[i][j] += val;
        }
    }   
}

int getSum(int x,int y)
{
    int sum=0;
    for(int i=x; i>0; i-=lowbit(i))
    {
        for(int j=y; j>0; j-=lowbit(j))
        {
            sum += p[i][j];
        }
    }
    return sum;
}