Ultraviolet A 1264-Binary Search Tree (BST)

Link to the question: Ultraviolet A 1264-Binary Search Tree

Given an insert sequence, the number of insert sequences must be output to make the generated BST the same.

Solution: the nature of combined mathematics + BST does not affect the nodes of the left and right subtree. So you can go up layer by layer.

#include <cstdio>#include <cstring>#include <algorithm>using namespace std;typedef long long ll;const int maxn = 20;const ll mod = 9999991;ll C[maxn+5][maxn+5];void get_C (int n) {    for (int i = 0; i <= n; i++) {        C[i][0] = C[i][i] = 1;        for (int j = 1; j < i; j++)            C[i][j] = (C[i-1][j-1] + C[i-1][j]) % mod;    }}struct BST {    int sz, cid[maxn+5][2];    int val[maxn+5], sum[maxn+5];    void init();    void insert(int& u, int v);    void pushup(int u);    ll count(int u);}AC;int main () {    get_C(maxn);    int cas, n, x;    scanf("%d", &cas);    while (cas--) {        int R = 0;        AC.init();        scanf("%d", &n);        for (int i = 0; i < n; i++) {            scanf("%d", &x);            AC.insert(R, x);        }        printf("%lld\n", AC.count(R));    }    return 0;}ll BST::count(int u) {    if (u == 0)        return 1;    ll ret = 1;    ret = (ret * count(cid[u][0])) % mod;    ret = (ret * count(cid[u][1])) % mod;    ret = (ret * C[sum[u]-1][sum[cid[u][0]]]) % mod;    return ret;}void BST::init() {    sz = 1;    sum[0] = val[0] = 0;    memset(cid[1], 0, sizeof(cid[1]));}void BST::pushup(int u) {    sum[u] = sum[cid[u][0]] + sum[cid[u][1]] + 1;    /*    sum[u] = 1;    if (cid[u][0])        sum[u] += sum[cid[u][0]];    if (cid[u][1])        sum[u] += sum[cid[u][1]];        */}void BST::insert(int& u, int v) {    if (u == 0) {        u = sz++;        memset(cid[u], 0, sizeof(cid[u]));        val[u] = v;        sum[u] = 1;        return;    }    if (val[u] < v)        insert(cid[u][1], v);    else        insert(cid[u][0], v);    pushup(u);}

Ultraviolet A 1264-Binary Search Tree (BST)