树状数组，二查索引树，BIT

//预处理//c[i]=a[i-lowbit(i)+1]+......a[i];for(int i=1;i<=n;i++)    for(int j=i-lowbit(i)+1;j<=i;j++)    c[i]+=a[j];//求和int sum(int x){    int ret=0;    while(x>0)    {        ret+=c[x];        x-=lowbit(x);    }    return ret;}//更新void update(int x,int d){    while(x<=n)    {        c[x]+=d;        x+=lowbit(x);    }}