单点修改，区间查询（求和）

单点更新，前缀最值

class TreeArray {
public: 
    static const int N = ::N;
    int val[N];
    int lowbit(int x) { return x & -x; }
    void upd(int u, int v) { for(int i = u; i < N; i += lowbit(i)) val[i] = max(val[i], v); }
    int query(int u) { int res = -inf; for(int i = u; i; i -= lowbit(i)) res = max(res, val[i]); return res; }
} t;