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算法提高之你能回答这些问题吗
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核心思想：线段树
- 用sum,lmax,rmax,tmax分别存线段长度,最大前缀,最大后缀,最大子段和
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#include <iostream>#include <cstring>#include <algorithm>using namespace std;const int N = 500010;int n,m;int w[N];struct Node{int l,r;int sum,lmax,rmax,tmax;}tr[N*4];void pushup(Node &u,Node &l,Node &r){u.sum = l.sum + r.sum;u.lmax = max(l.lmax,l.sum + r.lmax);u.rmax = max(r.rmax,r.sum + l.rmax);u.tmax = max(max(l.tmax,r.tmax) , l.rmax + r.lmax);}void pushup(int u){pushup(tr[u],tr[u<<1],tr[u<<1|1]);}void build(int u,int l,int r){if(l == r) tr[u] = {l,r,w[r],w[r],w[r],w[r]};else{tr[u] = {l,r};int mid = l+r>>1;build(u<<1,l,mid),build(u<<1|1,mid+1,r);pushup(u);}}void modify(int u,int x,int v){if(tr[u].l == x && tr[u].r == x) tr[u] = {x,x,v,v,v,v};else{int mid = tr[u].l+tr[u].r>>1;if(x <= mid) modify(u<<1,x,v);else modify(u<<1|1,x,v);pushup(u);}}Node query(int u,int l,int r){if(tr[u].l >= l && tr[u].r <= r) return tr[u];else{int mid = tr[u].l + tr[u].r >>1;if(r <= mid) return query(u<<1,l,r);else if(l >mid) return query(u<<1|1,l,r);else{auto left = query(u<<1,l,r);auto right = query(u<<1|1,l,r);Node res;pushup(res,left,right);return res;}}}int main(){cin>>n>>m;for(int i=1;i<=n;i++) cin>>w[i];build(1,1,n);int k,x,y;while (m -- ){cin>>k>>x>>y;if(k==1){if(x>y) swap(x,y);cout<<query(1,x,y).tmax<<endl;}else modify(1,x,y);}}