平衡树的入门题目，也可以用Treap来实现，我觉得Treap的核心就是那个rand()了用来保证树不退化成链，同时还是有平衡树的特性。。总体来说我觉得比较好些，而且比较快

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <cctype>
#include <iostream>
using namespace std;
const int MAXN = 500000;
struct node{int r, v, sz, number;node *ch[2];void rz(){sz = ch[0]->sz + ch[1]->sz + number;}
}Edges[MAXN+10], *ecnt = Edges, Tnull, *nil = &Tnull;
node *newnode(node *lch, node *rch, int v, int r){++ecnt;ecnt->ch[0] = lch;ecnt->ch[1] = rch;ecnt->v = v;ecnt->r = r;return ecnt;
}
node *retu;
int ret;
struct Treap{node *root;Treap(){root = nil; nil->ch[0] = nil->ch[1] = nil;}void rotate(node *&n, bool d){node *a = n->ch[d];n->ch[d] = a->ch[!d]; n->rz();a->ch[!d] = n; a->rz();n = a;}void Insert(node *&rt, int val){if(rt == nil){rt = newnode(nil, nil, val, rand());rt->number = 1;rt->rz();return ;}rt->sz++;if(val == rt->v){rt->number++;}else if(val < rt->v){Insert(rt->ch[0], val);if(rt->ch[0]->r < rt->r)rotate(rt, false);}else if(val > rt->v){Insert(rt->ch[1], val);if(rt->ch[1]->r < rt->r)rotate(rt, true);}}void del(node *&rt, int val){if(rt == nil) return ;rt->sz--;if(rt->v == val){if(rt->number > 1){rt->number--;}else if(rt->ch[0] == nil || rt->ch[1] == nil){if(rt->ch[0]!=nil)rt = rt->ch[0];elsert = rt->ch[1];}else{if(rt->ch[0]->r > rt->ch[1]->r){rotate(rt, false);del(rt->ch[1], val);rt->rz();}else{rotate(rt, true);del(rt->ch[0], val);rt->rz();}}}else if(val < rt->v)del(rt->ch[0], val);elsedel(rt->ch[1], val);}int Rank(node *rt, int val){if(rt == nil) return ret+1;if(rt->v == val)return ret+rt->ch[0]->sz+1;if(val < rt->v)return Rank(rt->ch[0], val);ret += rt->ch[0]->sz+rt->number;return Rank(rt->ch[1], val);}void pre(node *rt, int val){if(rt == nil) return ;else if(rt->v <= val){retu = rt;pre(rt->ch[1], val);}elsepre(rt->ch[0], val);}void bak(node *rt, int val){if(rt == nil) return ;else if(rt->v >= val){retu = rt;bak(rt->ch[0], val);}elsebak(rt->ch[1], val);}int order(node *rt, int rk){if(rt->sz < rk) return 0;if(rk <= rt->ch[0]->sz)return order(rt->ch[0], rk);else if(rk > rt->ch[0]->sz+rt->number)return order(rt->ch[1], rk-rt->ch[0]->sz-rt->number);return rt->v;}
}tp;
int main(){//freopen("data10.in", "r", stdin);int n;scanf("%d", &n);for(int i=0;i<n;i++){int o, x;scanf("%d %d", &o, &x);switch(o){case 1: tp.Insert(tp.root, x); break;case 2: tp.del(tp.root, x); break;case 3: ret = 0; printf("%d\n", tp.Rank(tp.root, x)); break;case 4: printf("%d\n", tp.order(tp.root, x)); break;case 5: retu = nil;tp.pre(tp.root, x-1); printf("%d\n", retu->v); break;case 6: retu = nil;tp.bak(tp.root, x+1); printf("%d\n", retu->v); break;}}return 0;
}