bzoj4900 [CTSC2017]密钥

传送门：http://www.lydsy.com/JudgeOnline/problem.php?id=4900

【题解】

恭喜bzoj达到40页

考场由于傻逼基数排序写挂了而gg。

竟然忘了考试前一天复习了kd-tree里面有nth_element这种东西。。

那么看看样例，找找规律就发现排序的关键字是前缀和尽量小，其次位置尽量靠后（对于第三种是靠前）

直接上nth_element就行了。。

# include <stdio.h>
# include <string.h>
# include <iostream>
# include <algorithm>
// # include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
const int M = 2e7 + ;
const int mod = 1e9+;

# define RG register
# define ST static

int n, k, S, seed;
int p[M], s[M];

struct pa {
    int s, pos;
    pa() {}
    pa(int s, int pos) : s(s), pos(pos) {}
}a[M];
int an = ;

inline bool cmp1(pa a, pa b) {
    return a.s<b.s || (a.s==b.s && a.pos>b.pos);
}
inline bool cmp2(pa a, pa b) {
    return a.s<b.s || (a.s==b.s && a.pos<b.pos);
} 

inline int randint() {
    seed = ((seed * ) ^ ) & ;
    return seed;
}

inline void input() {
    cin >> k >> seed >> S;
    int t = ;
    n = k *  + ;
    for(int i = ; i <= n; ++i) {
        p[i] = (randint() >> ) & ;
        t += p[i];
    }
    int i = ;
    while(t > k) {
        while(!p[i]) i++;
        p[i] = ;
        t--;
    }
    while(t < k) {
        while(p[i]) i++;
        p[i] = ;
        t++;
    }
}

int main() {
    input();
    for (int i=; i<=n; ++i) {
        if(p[i]) p[i] = -;
        else p[i] = ;
        s[i] = s[i-] + p[i];
        if(p[i] == ) {
            a[++an].s = s[i];
            a[an].pos = i;
        }
    }
    nth_element(a+, a+, a+an+, cmp1);
    cout << a[].pos << endl;
    nth_element(a+, a+S+, a+an+, cmp1);
    cout << a[S+].pos << endl;
    for(int i=; i<=an; ++i) a[i].s = -a[i].s;
    nth_element(a+, a+S+, a+an+, cmp2);
    cout << a[S+].pos << endl;
    return ;
}