CSES Dynamic Range Minimum Queries

Question

https://cses.fi/problemset/task/1649

I'm solving this problem using Segment Trees and the solution I've written is

#include <bits/stdc++.h>
#define MAX 1000000001

using namespace std; 

int n;  
vector<int> tree;

int sum(int a, int b)
{
    a += n; 
    b += n; 

    int s = INT_MAX; 
    while(a <= b) {
        if (a % 2 == 1) s = min(s, tree[a++]); 
        if (b % 2 == 0) s = min(s, tree[b--]); 

        a>>=1; 
        b>>=1; 
    }

    return s; 
}

void update(int k, int change)
{
    k += n; 
    tree[k] = change; 

    for(int i = k>>1; i >= 1; i>>=1) {
        tree[i] = min(tree[2*i], tree[2*i+1]); 
    }

    return; 
}

int main()
{
    int q; 
    cin >> n >> q; 

    n = pow(2, ceil(log2(n)));  
    tree.resize(2*n, INT_MAX); 
    for(int i = 0; i < n; i++) {
        cin >> tree[i+n]; 
    } 

    for(int i = n-1; i >= 1; i--) {
        tree[i] = min(tree[2*i], tree[2*i+1]); 
    }

    int type, a, b; 
    for(int i = 0; i < q; i++) {
        cin >> type >> a >> b; 
        if (type == 1) {
            update(a-1, b); 
        } else {
            cout << sum(a-1, b-1) << endl; 
        }
    }

    return 0; 
}

It works with first test case, but not with the second one. I've looked at other solutions online and they all look similar to mine. Please, help me spot the mistake.