前缀和与差分

二维前缀和

P2280 [HNOI2003]激光炸弹 

https://www.luogu.org/problemnew/show/P2280

FOR(i,1,n) 
{
    int x,y,v;
    cin>>x>>y>>v;
    s[x+1][y+1]=v;
}
FOR(i,1,5001) FOR(j,1,5001) s[i][j]+=s[i-1][j]+s[i][j-1]-s[i-1][j-1]; 
FOR(i,0,5000-r) FOR(j,0,5000-r)
    ans=max(ans,s[i+r][j+r]-s[i][j+r]-s[i+r][j]+s[i][j]);
cout<<ans;

差分：现修改，后询问，总复杂度O(n)