是一维树状数组的扩充，修改和查询的复杂度均为log^2(n). 空间复杂度略大 n^2
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;
}
