线段树水题

1.洛谷p1351 https://www.luogu.org/problem/show?pid=1531

//单点修改区间询问 
#include<iostream>
#include<cstdio>
#include<cstring>

#define maxn 200001

using namespace std;
int n,m,x,y,z,ans1,ans2;
char c;
struct node
{
    int l,r,sum,max;
}tree[maxn<<2];

inline int init()
{
    int x=0,f=1;char c=getchar();
    while(c>'9'||c<'0'){if(c=='-')f=-1;c=getchar();}
    while(c>='0'&&c<='9'){x=x*10+c-'0';c=getchar();}
    return x*f;
}

void build(int now,int l,int r)
{
    tree[now].l=l;tree[now].r=r;
    if(l==r)
    {
        tree[now].sum=tree[now].max=init();
        return;
    }
    int mid=(l+r)>>1;
    build(now<<1,l,mid);build(now<<1|1,mid+1,r);
    tree[now].sum=tree[now<<1].sum+tree[now<<1|1].sum;
    tree[now].max=max(tree[now<<1].max,tree[now<<1|1].max);
}

int query(int now,int l,int r)
{
    if(tree[now].l==l&&tree[now].r==r) return tree[now].max;
    int mid=(tree[now].l+tree[now].r)>>1;
    if(l>mid) return query(now<<1|1,l,r);
    else if(r<=mid) return query(now<<1,l,r);
    else return max(query(now<<1,l,mid),query(now<<1|1,mid+1,r));
}

int serch(int now,int pos)
{
    if(tree[now].l==pos&&tree[now].r==pos) return tree[now].sum;
    int mid=(tree[now].l+tree[now].r)>>1;
    if(pos<=mid) return serch(now<<1,pos);
    else if(pos>mid) serch(now<<1|1,pos);
}


void change(int now,int pos,int k)
{
    int l=tree[now].l;int r=tree[now].r;
    if(l==pos&&r==pos)
    {
        tree[now].sum=tree[now].max=k;
        return;
    }
    int mid=(tree[now].l+tree[now].r)>>1;
    if(pos>mid) change(now<<1|1,pos,k);
    if(pos<=mid) change(now<<1,pos,k);
    tree[now].sum=tree[now<<1].sum+tree[now<<1|1].sum;
    tree[now].max=max(tree[now<<1].max,tree[now<<1|1].max);
}

int main()
{
    n=init();m=init();
    build(1,1,n);
    for(int i=1;i<=m;i++)
    {
        cin>>c;x=init();y=init();
        if(c=='Q'){printf("%d\n",query(1,x,y));}
        else if(serch(1,x)<y) change(1,x,y);
    }
    return 0;
}