[树状数组]hdu 2155 Matrix

/**
[树状数组]hdu 2155 Matrix
二维树状数组，更新区间，查询点
详解见09年国家集训队论文 
http://www.cppblog.com/klion/archive/2010/05/25/116325.html?opt=admin
*/
#include <stdio.h>
#include <string.h>

#define N 1001
#define lowbit(i) (i) & (-i)
int mat[N][N],n;
void update(int x,int y)
{
    int i,j;
    for(i = x; i <= n; i += lowbit(i))
        for(j = y; j <= n; j += lowbit(j))
            mat[i][j] ^= 1;
}
int sum(int x,int y)
{
    int ans = 0,i,j;
    for(i = x; i > 0; i -= lowbit(i))
        for(j = y; j > 0; j -= lowbit(j))
            ans += mat[i][j];
    return ans;
}
int main()
{
    int t,q,x1,y1,