刚学平衡树，分别用了Splay和fhq-treap交了一遍。
这是Splay的板子，貌似比较短？

Splay

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>using namespace std;const int N = 100005, INF = 2e9;int n;
int nid, root;
int fa[N], ch[N][2];
int val[N];
int sz[N], rep[N];void update(int x) {sz[x] = sz[ch[x][0]]+sz[ch[x][1]]+rep[x];
}void rotate(int x) {int y = fa[x], z = fa[y];int d = ch[fa[x]][1]==x;ch[z][ch[fa[y]][1]==y] = x; fa[x] = z;ch[y][d] = ch[x][d^1]; fa[ch[x][d^1]] = y;ch[x][d^1] = y; fa[y] = x;update(y); update(x);
}void splay(int x, int goal) {for(; fa[x] != goal; rotate(x)) if(fa[fa[x]] != goal)(ch[fa[x]][0]==x)^(ch[fa[fa[x]]][0]==fa[x]) ? rotate(x) : rotate(fa[x]);if(!goal) root = x;
}void find(int v) {int u = root;if(!u) return ;while(ch[u][val[u]<v] && val[u] != v) u = ch[u][val[u]<v];splay(u, 0);
}int findPre(int v) {find(v);int u = root;if(val[u] < v) return u;u = ch[u][0];while(ch[u][1]) u = ch[u][1];return u;
}int findNxt(int v) {find(v);int u = root;if(val[u] > v) return u;u = ch[u][1];while(ch[u][0]) u = ch[u][0];return u;
}void insert(int v) {int u = root, f = 0;while(u && val[u] != v) f = u, u = ch[u][val[u]<v];if(u) rep[u]++;else {nid++;fa[nid] = f;ch[f][val[f]<v] = nid;val[nid] = v;sz[nid] = rep[nid] = 1;u = nid;}splay(u, 0);
}void del(int v) {int pre = findPre(v), nxt = findNxt(v);splay(pre, 0); splay(nxt, pre);int tar = ch[nxt][0];if(rep[tar] > 1) {rep[tar]--;splay(tar, 0);}else ch[nxt][0] = 0;
}int kth(int x) {if(sz[root] < x) return 0;int u = root;while(1) {int lson = ch[u][0];if(x <= sz[lson]) u = lson;else if(x > sz[lson]+rep[u]) x -= sz[lson]+rep[u], u = ch[u][1];else return val[u];}
}int main() {insert(-(1<<31)+2); //添加两个哨兵节点，防止越界insert((1<<31)-2);ios_base::sync_with_stdio(false);cin.tie(0); cout.tie(0);cin >> n;for(int i = 1, c, x; i <= n; i++) {cin >> c >> x;if(c == 1) insert(x);else if(c == 2) del(x);else if(c == 3) find(x), cout << sz[ch[root][0]] << endl;else if(c == 4) cout << kth(x+1) << endl;else if(c == 5) cout << val[findPre(x)] << endl;else if(c == 6) cout << val[findNxt(x)] << endl;}return 0;
}

这是fhq-treap的板子（为了美观，我已经不能再无脑压行了）

fhq-treap

#include <iostream>
#include <cstring>
#include <algorithm>
#include <ctime>using namespace std;const int N = 100005;int n;
int ch[N][2], val[N], sz[N], rnd[N];
int nid, root;void update(int x) {sz[x] = sz[ch[x][0]]+sz[ch[x][1]]+1;
}int merge(int x, int y) {if(!x || !y) return x+y;if(rnd[x] < rnd[y]) {ch[x][1] = merge(ch[x][1], y);update(x);return x;} else {ch[y][0] = merge(x, ch[y][0]);update(y);return y;}
}void split(int cur, int k, int &x, int &y) {if(!cur) x = y = 0;else {if(val[cur] <= k) x = cur, split(ch[cur][1], k, ch[cur][1], y);else y = cur, split(ch[cur][0], k, x, ch[cur][0]);update(cur);}
}int newNode(int v) {val[++nid] = v;sz[nid] = 1;rnd[nid] = rand();return nid;
}void insert(int v) {int x, y;split(root, v, x, y);root = merge(merge(x, newNode(v)), y);
}void del(int v) {int x, y, z;split(root, v, x, z); split(x, v-1, x, y);y = merge(ch[y][0], ch[y][1]);root = merge(merge(x, y), z);
}int rankOf(int v) {int x, y, ret;split(root, v-1, x, y);ret = sz[x]+1;root = merge(x, y);return ret;
}int kth(int src, int x) {if(sz[src] < x) return 0;int u = src;while (1) {if(x <= sz[ch[u][0]]) u = ch[u][0];else if(sz[ch[u][0]]+1 == x) return u;else if(sz[ch[u][0]]+1 < x) x -= sz[ch[u][0]]+1, u = ch[u][1];}
}int precursor(int v) { //前驱int x, y, ret;split(root, v-1, x, y);ret = val[kth(x, sz[x])];root = merge(x, y);return ret;
}int successor(int v) { //后继int x, y, ret;split(root, v, x, y);ret = val[kth(y, 1)];root = merge(x, y);return ret;
}int main() {srand((unsigned)time(NULL));ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);cin >> n;for(int i = 1, c, x; i <= n; i++) {cin >> c >> x;if(c == 1) insert(x);else if(c == 2) del(x);else if(c == 3) cout << rankOf(x) << endl;else if(c == 4) cout << val[kth(root, x)] << endl;else if(c == 5) cout << precursor(x) << endl;else if(c == 6) cout << successor(x) << endl;}return 0;
}