poj 3264 balanced lineup

#include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
using namespace std;
typedef long long ll;
#define maxn 50010
#define inf 0x3fffffff
int nmin , nmax;
struct node {
int l,r;
int nmin , nmax;
}tree[3*maxn];
int num[maxn];
void build (int i,int l,int r){
tree[i].l = l;
tree[i].r = r;
if (l==r){
tree[i].nmin = num[l];
tree[i].nmax = tree[i].nmin;
return;
}
else {
int mid = (l+r)/2;
build (2*i,l,mid);
build (2*i+1,mid+1,r);
tree[i].nmin = min (tree[i*2].nmin,tree[i*2+1].nmin);
tree[i].nmax = max (tree[i*2].nmax,tree[i*2+1].nmax);
}
}

void query (int i,int l,int r){

if (tree[i].l ==l && tree[i].r==r ){
nmin = min (nmin ,tree[i].nmin);
nmax = max (nmax ,tree[i].nmax);
return ;
}
int mid = (tree[i].l +tree[i].r)>>1;
if (r <= mid)query ( 2*i,l,r);
4000
else if (l>mid) query ( 2*i+1,l,r);
else {
query ( i*2 , l, mid);
query (2*i+1 , mid+1 ,r);
}
}

int main()
{
int n,q;
int l,r;
while (scanf ("%d %d",&n,&q)!=EOF){
for (int i=1;i<=n;i++){
scanf ("%d",&num[i]);
}
build (1,1,n);
for (int i =1;i<=q;i++){
scanf ("%d %d",&l,&r);
nmax = -inf;
nmin = inf;
query (1,l,r);
printf("%d\n", nmax - nmin);
}
}
return 0;
}