HDU 6231 (K-th Number)

題目連接：https://cn.vjudge.net/problem/HDU-6231

思路：二分+雙指針；

#include <stdio.h>
#include <iostream>
#include <string.h>
#include <math.h>
#include <algorithm>

using namespace std;
typedef long long int LL;


const int INF=2e9+1e8;
const int maxn=1e5+100;
int a[maxn],b[maxn];

int n,k;
LL m;
LL calc(int x) //計算區間第k大的數大於等於x的區間個數
{
    LL ans=0;
    int l=0,r=-1,num=0;
    while(r<n)
    {
        if(num<k)
        {
            if(a[r+1]>=x) num++;
            r++;
        }
        else 
        {
            if(num==k) ans+=n-r;
            if(a[l]>=x) num--;
            l++;
        }
    }
    return ans;
}
int main()
{
    int T;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d%d%lld",&n,&k,&m);
        for(int i=0;i<n;i++)
            scanf("%d",&a[i]),b[i]=a[i];
        sort(b,b+n);
        int len=unique(b,b+n)-b;
        int l=0,r=len-1,ans=-1;
        while(l<=r)
        {
            int mid=(l+r)>>1;
            if(calc(b[mid])>=m) ans=b[mid],l=mid+1;
            else r=mid-1;
        }
        printf("%d\n",ans);
    }
    return 0;
}