观察如下代码：

void upd(int x,int y){
    int u=b[x];
    while(x<=n){
        s[x]+=(y-u)%mod;
        s[x]=(s[x]+mod)%mod;
        sqs[x]+=(y*y%mod-u*u%mod)%mod;
        sqs[x]=(sqs[x]+mod)%mod;
        x+=lowbit(x);
    }
}

改为：

void upd(int x,int y){
    int u=b[x];
    int lst=x;
    while(x<=n){
        s[x]+=(y-u)%mod;
        s[x]=(s[x]+mod)%mod;
        sqs[x]+=(y*y%mod-u*u%mod)%mod;
        sqs[x]=(sqs[x]+mod)%mod;
        x+=lowbit(x);
    }b[lst]=y;
}