题目链接：点击打开链接

== 原来写1的时候已经把更新函数写好了。。

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <string.h>
#include <math.h>
#include <vector>
#include <map>
using namespace std;
#define N 50050
#define Lson(x) tree[x].l
#define Rson(x) tree[x].r
#define L(x) (x<<1)
#define R(x) (x<<1|1)
#define Sum(x) tree[x].sum
#define Max(x) tree[x].max
#define Lmax(x) tree[x].lmax
#define Rmax(x) tree[x].rmax
struct node{
    int l, r;
    int mid(){return (l+r)>>1;}
    int lmax, rmax, max, sum;
}tree[N<<2];
int n, a[N], Q;
void push_down(int id){}
void push_up(int id){
    Lmax(id) = max(Lmax(L(id)), Sum(L(id)) + Lmax(R(id)));
    Rmax(id) = max(Rmax(R(id)), Sum(R(id)) + Rmax(L(id)));
    Sum(id) = Sum(L(id)) + Sum(R(id));
    Max(id) = max(max(Max(L(id)), Max(R(id))), Rmax(L(id)) + Lmax(R(id)));
}
void updata_point(int val, int id){Lmax(id) = Rmax(id) = Max(id) = Sum(id) = val;}
void build(int l, int r, int id){
    Lson(id) = l; Rson(id) = r;
    if(l == r)
    {
        updata_point(a[l], id);
        return;
    }
    int mid = tree[id].mid();
    build(l, mid, L(id));
    build(mid+1, r, R(id));
    push_up(id);
}
void updata(int pos, int val, int id){
    push_down(id);
    if(Lson(id) == Rson(id))
    {
        updata_point(val, id);
        return ;
    }
    int mid = tree[id].mid();
    if(mid < pos)
        updata(pos, val, R(id));
    else
        updata(pos, val, L(id));
    push_up(id);
}
int query_l(int l, int r, int id){
    push_down(id);
    if(l == Lson(id) && Rson(id) == r) return Lmax(id);
    int mid = tree[id].mid();
    if(mid < l)
        return query_l(l, r, R(id));
    else if(r <= mid)
        return query_l(l, r, L(id));
    int lans = query_l(l, mid, L(id)), rans = query_l(mid+1, r, R(id));
    return max(lans, Sum(L(id)) + rans);
}
int query_r(int l, int r, int id){
    push_down(id);
    if(l == Lson(id) && Rson(id) == r) return Rmax(id);
    int mid = tree[id].mid();
    if(mid < l)
        return query_r(l, r, R(id));
    else if(r <= mid)
        return query_r(l, r, L(id));
    int lans = query_r(l, mid, L(id)), rans = query_r(mid+1, r, R(id));
    return max(rans, Sum(R(id)) + lans);
}
int query(int l, int r, int id){
    push_down(id);
    if(l == Lson(id) && Rson(id) == r)return Max(id);
    int mid = tree[id].mid();
    if(mid < l)
        return query(l, r, R(id));
    else if(r<=mid)
        return query(l, r, L(id));
    int lans = query(l, mid, L(id)), rans = query(mid+1, r, R(id));
    int ans = max(lans, rans);
    return max(ans, query_r(l, mid, L(id)) + query_l(mid+1, r, R(id)));
}
int main(){
    while(~scanf("%d",&n)){
        for(int i = 1; i <= n; i++)scanf("%d",&a[i]);
        build(1, n, 1);
        scanf("%d",&Q);
        while(Q--){
            int l, r, op;
            scanf("%d %d %d", &op, &l, &r);
            if(op == 1)
                printf("%d\n", query(l, r, 1));
            else
                updata(l, r, 1);
        }
    }
    return 0;
}
/*
3
-1 2 3
1
1 2

*/

Spoj 1716 Can you answer these queries III 线段树 单点修改 区间求最大子段和