Note -「模板」FHQ-Treap

// Fhq-Treap

const int MAXN = 1e5 + 5;

struct Fhq_Treap {
    struct Fhq_Node {
        int l, r, val, key, size;
#define lson tr[p].l
#define rson tr[p].r
        Fhq_Node() {}
        Fhq_Node(int L, int R, int Val, int Key, int Size) {
            l = L;
            r = R;
            val = Val;
            key = Key;
            size = Size;
        }
    } tr[MAXN];
    int tot, root;

    Fhq_Treap() {
        tot = 0, root = 0;
    }

    int Get(int val) {
        tr[++tot] = Fhq_Node(0, 0, val, rand(), 1);
        return tot;
    }

    void Push_Up(int p) { tr[p].size = tr[lson].size + tr[rson].size + 1; }

    void Split(int p, int val, int &x, int &y) {
        if (!p) {
            x = 0, y = 0;
            return;
        }
        if (tr[p].val <= val) {
            x = p;
            Split(rson, val, rson, y);
        } else {
            y = p;
            Split(lson, val, x, lson);
        }
        Push_Up(p);
    }

    int Merge(int x, int y) {
        if (!x || !y)
            return x + y;
        if (tr[x].key <= tr[y].key) {
            tr[x].r = Merge(tr[x].r, y);
            Push_Up(x);
            return x;
        } else {
            tr[y].l = Merge(x, tr[y].l);
            Push_Up(y);
            return y;
        }
    }

    void Insert(int val) {
        int x, y;
        Split(root, val, x, y);
        root = Merge(Merge(x, Get(val)), y);
    }

    void Delete(int val) {
        int x, y, z;
        Split(root, val, x, z);
        Split(x, val - 1, x, y);
        y = Merge(tr[y].l, tr[y].r);
        root = Merge(Merge(x, y), z);
    }

    int Get_Rank(int val) {
        int x, y, ret;
        Split(root, val - 1, x, y);
        ret = tr[x].size + 1;
        root = Merge(x, y);
        return ret;
    }

    int Get_Val(int Rank) {
        int p = root;
        while (p) {
            if (tr[lson].size + 1 == Rank)
                return tr[p].val;
            else if (Rank <= tr[lson].size)
                p = lson;
            else {
                Rank -= (tr[lson].size + 1);
                p = rson;
            }
        }
        return 0;
    }

    int Get_Pre(int val) {
        int x, y, p;
        Split(root, val - 1, x, y);
        p = x;
        while (rson) p = rson;
        int ret = tr[p].val;
        root = Merge(x, y);
        return ret;
    }

    int Get_Next(int val) {
        int x, y, p;
        Split(root, val, x, y);
        p = y;
        while (lson) p = lson;
        int ret = tr[p].val;
        root = Merge(x, y);
        return ret;
    }
#undef lson
#undef rson
} tree;