区间中最大数（线段树入门）

http://www.51nod.com/onlineJudge/questionCode.html#!problemId=1174

第一次学线段树，感觉脑子里一片浆糊

#include<iostream>
#include<algorithm>
using namespace std;
typedef long long ll;
const int MAXN=1e4+5;

struct node 
{
    ll l,r;
    ll val;
}tree[4*MAXN];
int a[MAXN];

void build(int n,ll left,ll right)
{
    tree[n].l=left;
    tree[n].r=right;
    if(left==right)
        tree[n].val=a[left];
    else
    {
        ll mid=(left+right)/2;
        build(2*n,left,mid);
        build(2*n+1,mid+1,right);
        tree[n].val=max(tree[2*n].val,tree[2*n+1].val);
    }
}
ll ask(int n,ll left,ll right)
{
    if(tree[n].l==tree[n].r)
        return tree[n].val;
    else
    {
        ll mid=(tree[n].r+tree[n].l)/2;
        if(right<=mid)
            return ask(n*2,left,right);
        else if(left>mid)
            return ask(n*2+1,left,right);
        else
            return max(ask(n*2,left,mid),ask(n*2+1,mid+1,right));
    }
}
int main()
{
    ll n,m,left,right;
    
    cin>>n;
    for(int i=1;i<=n;i++)
        cin>>a[i];
    build(1,1,n);
    cin>>m;
    for(int i=1;i<=m;i++)
    {
        cin>>left>>right;
        ll ans=ask(1,left+1,right+1);
        cout<<ans<<endl;
    }
    return 0;
}