Hello :)

I'm trying to solve the KGSS problem on SPOJ . I implemented a segment tree data structure , and wrote a simple brute force to test my answers against. However , I keep getting WA. For any interval represented by a segment tree node , res[node] is the sum of highest two integers on that interval , and f[node] is the largest integer on that interval. Can anyone help ? Thanks a lot.

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

const int MAXN = 400004;
int N , Q;
int res[MAXN];
int f[MAXN];
int x[MAXN];

struct qstruct { 
	int a;
	int b;
	qstruct(int u , int v){
		a = u;
		b = v;
	}
};

void update(int node , int s , int e , int qs , int qe , int val){
	if(s > qe || e < qs)
		return;
	if(s >= qs && e <= qe){
		f[node] = val;
		return;
	}
	update(2 * node , s , (s + e)/2 , qs , qe , val);
	update(2 * node + 1 , (s + e)/2 + 1 , e , qs , qe , val);
	f[node] = max(f[2 * node] , f[2 * node + 1]);
	res[node] = max(max(res[2 * node] , res[2 * node + 1]) , f[2 * node] + f[2 * node + 1]);
	return;
}
qstruct query(int node , int s , int e , int qs , int qe){
	if(s > qe || e < qs)
		return qstruct(0 , 0);
	if(s >= qs && e <= qe)
		return qstruct(res[node] , f[node]);
	qstruct q1 = query(2 * node , s , (s+e)/2 , qs , qe);
	qstruct q2 = query(2 * node + 1 , (s+e)/2+1 , e , qs , qe);
	qstruct q = qstruct(max(max(q1.a , q2.a) , q1.b + q2.b) , max(q1.b , q2.b));
	return q;
}
void build_segment_tree(int node , int s , int e){
	if(s == e){
		f[node] = x[s];
		return;
	}
	build_segment_tree(2 * node , s , (s + e) / 2);
	build_segment_tree(2 * node + 1 , (s + e ) / 2 + 1 , e);
	res[node] = max(max(res[2 * node] , res[2 * node + 1]) , f[2 * node] + f[2 * node + 1]);
	f[node] = max(f[2 * node] , f[2 * node + 1]);
	return;
}
int main(){
	ios::sync_with_stdio(false);
	scanf("%d" , &N);
	for(int n = 1 ; n <= N ; n++)
		scanf("%d" , &x[n]);
	build_segment_tree(1 , 1 , N);

	scanf("%d" , &Q);
	for(int q = 1 ; q <= Q ; q++){
		char qtype; 
		int u , v;
		cin >> qtype;
		if(qtype == 'U'){
			int idx , val;
			scanf("%d %d" , &idx , &val);x[idx]=val;
			update(1 , 1 , N , idx , idx , val);
		}
		else {
			int u , v;
			scanf("%d %d" , &u , &v);
			printf("%d\n" , query(1 , 1 , N , u , v).a);
			//int ans = 0;
			//for(int i = u ; i < v ; i++)
				//for(int j = i + 1  ; j<=v;j++)
					//ans=max(ans,x[i]+x[j]);
			//printf("BRUTE FORCE :%d\n",ans);
		}
	}
	return 0;
}