RMQ 模板题，求区间最大值，求助

#include<iostream>
#include<cmath>
#include<algorithm>
#ifndef ONLINE_JUDGE		// 洛谷判断
	#pragma GCC optimize(2)	// 火车头 (O2)
#endif
using namespace std;
const int N = 1e5 + 5;
int st[N][30], a[N], lg[N], n, m;
void preLg()	// 预处理 ∑{ i = 1, i <= n : log2(i) }
{
	lg[1] = 0;
	for ( int i = 2; i <= n; i++ ) { lg[i] = lg[i / 2] + 1; }
}
void preST()	// 初始化 ST 表
{
	for ( int i = 1; i <= n; i++ ) { st[i][0] = a[i]; }
	for ( int j = 1; j <= ( int ) lg[n]; j++ ) for ( int i = 1; i <= n; i++ ) { st[i][j] = max ( st[i][j - 1], st[i + ( 1 << ( j - 1 ) )][j - 1] ); }
}
int query ( int x, int y )
{
	int len = lg[y - x + 1];
	return st[x][len - 1] + st[y - ( 1 << ( len - 1 ) )][len - 1];
}
int main()
{
	cin >> n >> m;
	preLg();
	for ( int i = 1; i <= n; i++ ) { cin >> a[i]; }
	preST();
	for ( int x, y, i = 0; i < m; i++ )
	{
		cout << query ( x, y ) << endl;
	}
	return 0;
}