Down Segment Tree RMQ FAST

#include <bits/stdc++.h>

using namespace std;

const int T = (1 << 21);
int n;

int tree[T];
int arr[T];

void build() {
    for (int i = 0; i < n; i++) {
        tree[i + n] = arr[i];
    }
    for (int i = n - 1; i >= 0; i--) {
        tree[i] = min(tree[i << 1], tree[i << 1 | 1]);
    }
}

void update(int pos, int val) {
    pos += n;
    tree[pos] = val;
    for (tree[pos] = val; pos > 1; pos >>= 1) {
        tree[pos >> 1] = min(tree[pos], tree[pos ^ 1]);
    }
}

int find_min(int ql, int qr) {
    int ans = numeric_limits<int>::max();
    ql += n;
    qr += n;
    while (ql < qr) {
         if (ql & 1) {
             ans = min(ans, tree[ql]);
             ql++;
         }
         if (qr & 1) {
             qr--;
             ans = min(ans, tree[qr]);
         }
         ql >>= 1;
         qr >>= 1;
    }
    return ans;
}

int main() {
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> arr[i];
    }
    build();
    int q;
    cin >> q;
    while (q--) {
        char c;
        cin >> c;
        if (c == 'u') { // update
            int pos, val;
            cin >> pos >> val;
            pos--; // 0-indexing
            update(pos, val);
        } else { // get
            int ql, qr;
            cin >> ql >> qr;
            ql--; // 0-indexing, half-intervals
            cout << find_min(ql, qr) << endl;
        }
    }
}