P1627 [CQOI2009]中位数

#include <bits/stdc++.h>
#define ios ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0)
#define debug_in  freopen("in.txt","r",stdin)
#define deubg_out freopen("out.txt","w",stdout);
#define pb push_back
#define all(x) x.begin(),x.end()
#define fs first
#define sc second
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
const int maxn = 1e6+10;
const int maxM = 1e6+10;
const int inf = 1e8;
const ll inf2 = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9+7; 

template<class T>void read(T &x){
    T s=0,w=1;
    char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')w=-1;ch=getchar();}
    while(ch>='0'&&ch<='9') s=s*10+ch-'0',ch=getchar();
    x = s*w;
}
template<class H, class... T> void read(H& h, T&... t) {
    read(h);
    read(t...);
}

void pt() { cerr << endl; }
template<typename Head, typename... Tail> void pt(Head H, Tail... T) { cerr << ' ' << H; pt(T...); }

前缀和，后缀和问题
//--------------------------------------------

int N,b;
int a[maxn];
int mpL[maxn];
int id = -1;
void solve(){
    mpL[0 + 100000] = 1;
    int sum = 0;
    for(int i = id-1;i>=1;i--){
        if(a[i] > b) sum++;
        else sum--;
        mpL[sum + 100000]++;
    }
    int ans = 0;
    sum = 0;
    for(int i = id;i<=N;i++){
        if(a[i] > b) sum++;
        if(a[i] < b) sum--;
        ans += mpL[-sum + 100000];
    }
    printf("%d\n",ans);
}
int main(){
//    debug_in;

    read(N,b);
    for(int i = 1;i<=N;i++) {
        read(a[i]);
        if(a[i] == b) id = i;
    }
    solve();
    return 0;
}