Segtree-Style Binary Search

struct Segtree //max with binary searching
{
    ll tr[4000001];

    void build(int lo, int hi, int cur)
    {
        if (lo == hi)
        {
            tr[cur] = pre[lo];
            return;
        }
        int mid = (lo+hi)/2;
        build(lo, mid, cur*2);
        build(mid+1, hi, cur*2+1);
        tr[cur] = max(tr[cur*2], tr[cur*2+1]);
    }

    void build() { build(1, n, 1); }

    int find(ll k, int lo, int hi, int cur) //find lowest pos such that a[pos] >= k
    {
        if (lo == hi && tr[cur] >= k) return lo;
        int mid = (lo+hi)/2;
        if (tr[cur*2] >= k) return find(k, lo, mid, cur*2); //prioritize left (lower index)
        else if (tr[cur*2+1] >= k) return find(k, mid+1, hi, cur*2+1);
        else return -1;
    }

    int find(ll k) { return find(k, 1, n, 1); } //logn

};