RMQ求区间最大最小值

#include<iostream>
#include<cmath>
#include<cstdio>
#define N 50005
using namespace std;
int maxx[N][20],minn[N][20],a[N];
int ST(int n)
{
    for(int i=1;i<=n;i++)
    {
        maxx[i][0]=a[i];
        minn[i][0]=a[i];
    }
    for(int j=1;(1<<j)<=n;j++)
    {
        for(int i=1;i+(1<<j)-1<=n;i++)
        {
            maxx[i][j]=max(maxx[i][j-1],maxx[i+(1<<(j-1))][j-1]);
            minn[i][j]=min(minn[i][j-1],minn[i+(1<<(j-1))][j-1]);
        }
    }
}
int main()
{
    int n,q,lf,rg,mx,mn;
    cin>>n>>q;
    for(int i=1;i<=n;i++)
    {
        scanf("%d",&a[i]);
    }
    ST(n);
    for(int i=1;i<=q;i++)
    {
        scanf("%d %d",&lf,&rg);
        int k=log2(rg-lf+1);
        mx=max(maxx[lf][k],maxx[rg-(1<<k)+1][k]);
        mn=min(minn[lf][k],minn[rg-(1<<k)+1][k]);
        cout<<mx-mn<<endl;
    }
}