模板——线段树维护最大子段和 SP1716 GSS3

${color{Pink}{>>Question}}$

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>
#define ll long long
using namespace std;

template <typename T> void in(T &x) {
    x = 0; T f = 1; char ch = getchar();
    while(!isdigit(ch)) {if(ch == '-') f = -1; ch = getchar();}
    while(isdigit(ch)) {x = 10*x + ch - '0'; ch = getchar();}
    x *= f;
}

template <typename T> void out(T x) {
    if(x < 0) putchar('-'),x = -x;
    if(x > 9) out(x/10);
    putchar(x%10 + '0');
}

//------------------------------------------------------------

const int N = 50005;

int n,q,a;

struct node {
    int lm,rm,all,sum;
}t[N<<2];

void P_up(node &u,node ls,node rs) {
    u.sum = ls.sum + rs.sum;
    u.all = max(max(ls.all,rs.all),ls.rm+rs.lm);
    u.lm = max(ls.lm,ls.sum+rs.lm); u.rm = max(rs.rm,rs.sum+ls.rm);
    u.all = max(u.all,max(u.lm,u.rm));
}

void Build(int u,int l,int r) {
    if(l == r) {in(a); t[u].lm = t[u].rm = t[u].all = t[u].sum = a; return;}
    int mid = (l+r)>>1;
    Build(u<<1,l,mid); Build(u<<1|1,mid+1,r);
    P_up(t[u],t[u<<1],t[u<<1|1]);
}

void A(int u,int l,int r,int x,int k) {
    if(l == r) {t[u].all = t[u].lm = t[u].rm = t[u].sum = k; return;}
    int mid = (l+r)>>1;
    if(x <= mid) A(u<<1,l,mid,x,k); else A(u<<1|1,mid+1,r,x,k);
    P_up(t[u],t[u<<1],t[u<<1|1]);
}

node Q(int u,int l,int r,int x,int y) {
    if(x <= l && y >= r) {return t[u];}//important
    int mid = (l+r)>>1;
    if(y <= mid) return Q(u<<1,l,mid,x,y);
    else if(x > mid) return Q(u<<1|1,mid+1,r,x,y);
    else {
        node _u,ls,rs;
        ls = Q(u<<1,l,mid,x,mid); rs = Q(u<<1|1,mid+1,r,mid+1,y);
        P_up(_u,ls,rs);
        return _u;
    }
}

int main() {
    int op,x,y; in(n);
    Build(1,1,n);
    in(q);
    while(q--) {
        in(op); in(x); in(y);
        if(!op) A(1,1,n,x,y);
        else out(Q(1,1,n,x,y).all),putchar('
');
    }
    return 0;
}