Revise paste Untitled

#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <cstring>
#include <algorithm>
#include <stack>
#include <queue>

using namespace std;
const int maxn = 1e5 + 10;
const int INF = 2e9;
int arr[maxn];
pair<int, int> segment_tree[3 * maxn];

void merge_nodes(pair<int, int> & tree_node, pair<int, int> & L, pair<int, int> & R) {
    vector<int> v;
    v.push_back(L.first);
    v.push_back(L.second);
    v.push_back(R.first);
    v.push_back(R.second);
    
    sort(v.begin(), v.end());
    tree_node = make_pair(v[2], v[3]);
}
pair<int, int> merge_query(pair<int, int> L, pair<int, int> R) {
    vector<int> v;
    v.push_back(L.first);
    v.push_back(L.second);
    v.push_back(R.first);
    v.push_back(R.second);
    
    sort(v.begin(), v.end());
    
    return make_pair(v[2], v[3]);
}
void build_tree(int L, int R, int position) {
    if(L == R) {
        segment_tree[position] = make_pair(arr[L], -INF);
    }
    else {
        int middle = (L + R) / 2;
        build_tree(L, middle, 2 * position);
        build_tree(middle + 1, R, 2 * position + 1);
        merge_nodes(segment_tree[position], segment_tree[2 * position], segment_tree[2 * position + 1]);
    }
}
// L R i L R j L R
pair<int, int> query(int L, int R, int position, int i, int j) {
    if(i <= L and R <= j) {
        return segment_tree[position];
    }
    if(R < i or j < L) {
        return make_pair(-INF, -INF);
    }
    int middle = (L + R) / 2;
    return merge_query(query(L, middle, 2 * position, i, j), query(middle + 1, R, 2 * position + 1, i, j));
}
void update(int L, int R, int position, int idx, pair<int, int> new_value) {
    if(L == R) {
        segment_tree[position] = new_value;
        return;
    }
    int middle = (L + R) / 2;
    if(idx <= middle) {
        update(L, middle, 2 * position, idx, new_value);
    }
    else {
        update(middle + 1, R, 2 * position + 1, idx, new_value);
    }
    merge_nodes(segment_tree[position], segment_tree[2 * position], segment_tree[2 * position + 1]);
}
int main() {
    ios_base::sync_with_stdio(false);
    int n;
    cin >> n;
    
    for(int i = 0; i < n; i++) {
        cin >> arr[i];
    }
    build_tree(0, n - 1, 1);
    
    int q;
    cin >> q;
    
    for(int i = 0; i < q; i++) {
        char c;
        cin >> c;
        
        if(c == 'U') {
            int idx, new_value;
            cin >> idx >> new_value;
            idx--;
            update(0, n - 1, 1, idx, make_pair(new_value, -INF));
        }
        else {
            int a, b;
            cin >> a >> b;
            a--; b--;
            
            pair<int, int> p = query(0, n - 1, 1, a, b);
            
            cout << p.first + p.second << endl;
        }
    }
    
    return 0;
}