ACM学习历程—51NOD 1685 第K大区间2(二分 && 树状数组 && 中位数)

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <set>
#include <map>
#include <queue>
#include <vector>
#include <string>
#define LL long long

using namespace std;

//线段树
//区间每点增值，求区间和
const int maxN = 100010;

LL d[2][maxN*2];

int lowbit(int x)
{
    return x&(-x);
}

void add(int t, int id,int pls)
{
    while(id <= maxN<<1)//id最大是maxN
    {
        d[t][id] += pls;
        id += lowbit(id);
    }
}

LL sum(int t, int to)
{
    LL s = 0;
    while(to > 0)
    {
        s = s + d[t][to];
        to -= lowbit(to);
    }
    return s;
}

LL query(int t, int from, int to)
{
    return sum(t, to) - sum(t, from-1);
}

int n, a[maxN], b[maxN];
LL k;

LL judge(int t)
{
    for (int i = 0; i < n; ++i)
    {
        if (a[i] > t) b[i] = 1;
        else if (a[i] == t) b[i] = 0;
        else b[i] = -1;
    }
    memset(d, 0, sizeof(d));
    int sum = 0;
    LL ans = 0;
    for (int i = 0; i < n; ++i)
    {
        sum += b[i];
        ans += query(!(i%2), 100005-n, 100005+sum-1);
        if (i%2 == 0 && sum > 0) ans++;
        add(i%2, 100005+sum, 1);
    }
    return ans;
}

void work()
{
    int lt, rt, mid;
    lt = rt = a[0];
    for (int i = 1; i < n; ++i)
    {
        lt = min(lt, a[i]);
        rt = max(rt, a[i]);
    }
    while ((LL)lt+1 < rt)
    {
        mid = ((LL)lt+rt)>>1;
        if (judge(mid) > k-1) lt = mid;
        else rt = mid;
    }
    if (judge(lt) <= k-1) printf("%d\n", lt);
    else printf("%d\n", rt);
}

int main()
{
    //freopen("test.in", "r", stdin);
    while (scanf("%d", &n) != EOF)
    {
        cin >> k;
        for (int i = 0; i < n; ++i) scanf("%d", &a[i]);
        work();
    }
    return 0;
}