线段树+Lazy标记（我的模版）

 #include <bits/stdc++.h>
 using namespace std;
 typedef long long ll;
 typedef unsigned long long ull;
 #define INF 0X3f3f3f3f
 const ll MAXN = 2e5 + ;
 const ll MOD = 1e9 + ;
 ll a[MAXN];
 struct node
 {
     int left, right; //区间端点
     ll max, sum;     //看题目要求
     ll lazy;
     void update(ll x)
     {
         sum += (right - left + ) * x;
         lazy += x;
     }
 } tree[MAXN << ];
 //建树，开四倍
 void push_up(int x)
 {
     tree[x].sum = tree[x << ].sum + tree[x <<  | ].sum;
     tree[x].max = max(tree[x << ].max, tree[x <<  | ].max);
 }
 void push_down(int x)
 {
     ll lazeval = tree[x].lazy;
     if (lazeval)
     {
         tree[x << ].update(lazeval);
         tree[x <<  | ].update(lazeval);
         tree[x].lazy = ;
     }
 }
 //若原数组从a[1]~a[n]，调用build(1,1,n);
 void build(int x, int l, int r)
 {
     tree[x].left = l;
     tree[x].right = r;
     tree[x].sum = tree[x].lazy = ;
     if (l == r) //若到达叶节点
     {
         tree[x].sum = a[l]; //存储A数组的值
         tree[x].max = a[l];
     }
     else
     {
         int mid = (l + r) / ;
         build(x << , l, mid);
         build(x <<  | , mid + , r);
         push_up(x);
     }
     return;
 }
 //update(1,l,r,v)
 void update(int x, int l, int r, ll v) //区间更新
 {
     int L = tree[x].left, R = tree[x].right;
     if (l <= L && R <= r)
     {
         tree[x].update(v);
         tree[x].max += v;
     }
     else
     {
         push_down(x);
         int mid = (L + R) / ;
         if (mid >= l)
             update(x << , l, r, v);
         if (r > mid)
             update(x <<  | , l, r, v);
         push_up(x);
     }
 }
 //单点更新,更新pos点，调用add(1,pos,v)
 void add(int x, int pos, ll v) //当前更新的节点的编号为id（一般是以1为第一个编号）。pos为需要更新的点的位置，v为修改的值的大小
 {

     int L = tree[x].left, R = tree[x].right;
     if (L == pos && R == pos) //左右端点均和pos相等，说明找到了pos所在的叶子节点
     {
         tree[x].sum += v;
         tree[x].max += v;
         return; //找到了叶子节点就不需要在向下寻找了
     }
     int mid = (L + R) / ;
     if (pos <= mid)
         add(x << , pos, v);
     else
         add(x <<  | , pos, v); //寻找k所在的子区间
     push_up(x);                  //向上更新
 }
 //调用query_sum（1,l,r)即可查询[l,r]区间内元素的总和
 //区间查询
 ll query_sum(int x, int l, int r)
 {
     int L = tree[x].left, R = tree[x].right;
     if (l <= L && R <= r)
         return tree[x].sum;
     else
     {
         push_down(x);
         ll ans = ;
         int mid = (L + R) / ;
         if (mid >= l)
             ans += query_sum(x << , l, r);
         if (r > mid)
             ans += query_sum(x <<  | , l, r);
         push_up(x);
         return ans;
     }
 }
 //调用query_max(1,l,r)即可求[l,r]区间内元素的最大值
 ll query_max(int x, int l, int r)
 {
     int L = tree[x].left, R = tree[x].right;
     if (l == L && R == r)
         return tree[x].max;
     else
     {
         push_down(x);
         int mid = (L + R) / ;
         if (r <= mid)
             return query_max(x << , l, r);
         else if (l > mid)
             return query_max(x <<  | , l, r);
         else
             return max(query_max(x << , l, mid), query_max(x <<  | , mid+, r));
     }
 }
 int main()
 {
     ll n, m;
     while (scanf("%lld%lld", &n, &m) != EOF)
     {
         for (int i = ; i <= n; i++)
             scanf("%lld", &a[i]);
         build(, , n);
         getchar();
         while (m--)
         {
             int l, r;
             char que;
             scanf("%c %d %d", &que, &l, &r);
             getchar();
             if (que == 'Q')
                 printf("%lld\n", query_max(, l, r));
             else if (que == 'U')
                 update(, l,l, r - a[l]),a[l]=r;
         }
     }
     return ;
 }