1270. 数列区间最大值

思路：
根据线段树的思路，可以将线段树的另一个数字用maxn来表示，每次更新，去左右儿子的最大值覆盖最大值
代码：

# include<iostream>
# include<cstdio>
# include<cstring>
# include<algorithm>
using namespace std;

const int N = 100010;
int w[N];
int n,m;

struct node
{
    
    
    int l,r;
    int maxn;
}tr[4 * N];

void pushup(int u)
{
    
    
    tr[u].maxn = max(tr[u << 1].maxn,tr[u << 1 | 1].maxn);
}

void build(int u,int l,int r)
{
    
    
    if(l == r)
    tr[u] = {
    
    l,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);
    }
}

int query(int u,int l,int r)
{
    
    
    if(tr[u].l >= l && tr[u].r <= r) return tr[u].maxn;
    int mid = tr[u].l + tr[u].r >> 1;
    int maxx = 0;
    if(l <= mid) maxx = max(query(u << 1,l,r),maxx);
    if(r > mid) maxx = max(query(u << 1 | 1,l,r),maxx);
    return maxx;
}

int main()
{
    
    
    int x,y;
    scanf("%d %d",&n,&m);
    for(int i = 1;i <= n;i++)
    {
    
    
        scanf("%d",&w[i]);
    }
    build(1,1,n);
    while(m--)
    {
    
    
        scanf("%d %d",&x,&y);
        printf("%d\n",query(1,x,y));
    }
    
    return 0;
}