差分模版题(需理解才明白)AT2442 フェーン現象 (Foehn Phenomena)

https://www.luogu.org/problemnew/show/AT2442

 #include <bits/stdc++.h>
 #define read read()
 #define up(i,l,r) for(register int i = (l);i <= (r);i++)
 #define down(i,l,r) for(register int i = (l);i >= (r);i--)
 #define traversal_vedge(i) for(register int i = head[u]; i ;i = e[i].nxt)
 #define ll long long
 using namespace std;
 int read
 {
     int x = , f = ; char ch = getchar();
     while(ch <  || ch > ) {if(ch == '-')f = -; ch = getchar();}
     while(ch >= && ch <=) {x =  * x + ch - ;ch = getchar();}
     return x * f;
 }
 const int N =  2e5+;
 ll n,q,S,T,b[N],ans;

 inline ll get(ll x)
 {
     return x >  ? (-x*S) : (-x*T);
 }

 int main()
 {
     freopen("input.txt","r",stdin);
     n = read; q = read; S = read; T = read;
     ll last = read;
     up(i,,n)
     {
         ll x = read;
         b[i] = x - last;
         last = x;
         ans += get(b[i]);
     }
     //ans代表b[i](1~n)遍历后的答案;
     while(q--)
     {
         int l = read, r = read, x = read;
         ans -= get(b[l]);// b[l]改变,先减后加;
         b[l] += x;
         ans += get(b[l]);
         if(r < n)
         {
             ans -= get(b[r+]);//debug -= -> +=
             b[r+] -= x;
             ans += get(b[r+]);//degug += -> -=
         }
         printf("%lld\n",ans);
     }
     return ;
 }