[luoguP1168]中位数（主席树+离散化）

传送门

模板题一道，1A。

——代码

 #include <cstdio>
 #include <algorithm>
 #define ls son[now][0], l, mid
 #define rs son[now][1], mid + 1, r

 using namespace std;

 const int N = ;

 int n, sz, tot;
 int a[N], b[N], sum[ * N], rt[ * N], son[ * N][];

 inline void build(int &now, int l, int r)
 {
     now = ++tot;
     if(l == r) return;
     int mid = (l + r) >> ;
     build(ls);
     build(rs);
 }

 inline void update(int &now, int l, int r, int last, int x)
 {
     now = ++tot;
     sum[now] = sum[last] + ;
     son[now][] = son[last][];
     son[now][] = son[last][];
     if(l == r) return;
     int mid = (l + r) >> ;
     if(x <= mid) update(ls, son[now][], x);
     else update(rs, son[now][], x);
 }

 inline int query(int now, int l, int r, int x)
 {
     if(l == r) return l;
     int mid = (l + r) >> , cnt = sum[son[now][]];
     if(x <= cnt) return query(ls, x);
     else return query(rs, x - cnt);
 }

 int main()
 {
     int i, k;
     scanf("%d", &n);
     for(i = ; i <= n; i++) scanf("%d", &a[i]), b[i] = a[i];
     sort(b + , b + n + );
     sz = unique(b + , b + n + ) - (b + );
     build(rt[], , sz);
     for(i = ; i <= n; i++)
     {
         k = lower_bound(b + , b + sz + , a[i]) - b;
         update(rt[i], , sz, rt[i - ], k);
         if(i %  == ) printf("%d\n", b[query(rt[i], , sz, (i + ) >> )]);
     }
     return ;
 }