二维数组数组

二维树状数组:

add: (x,y)位置的元素值加d;
sum:查询(1,1)~(x,y)子矩阵元素和;

注意树状数组的元素，下标都要从1开始。

void add(int x, int y, int d) {
    int i, j;
    for(i = x; i < N; i += lowbit(i))
        for(j = y; j < N; j += lowbit(j))
            mat[i][j] += d;
}

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