1370D - Odd-Even Subsequence - CodeForces Solution

#include <iostream>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <map>
#include <vector>
#include <set>
#include <algorithm>
#include <sstream>
#include <queue>
#include <cctype>
#include <cmath>
#include <fstream>

using namespace std;

typedef long long ll;
#define REP(i,k) for(int (i)=0;(i)<(k);(i)++)
#define INF ~(1<<31)
#define CLR(a) memset((a),0,sizeof((a)))
#define EPS 1e-9
#define DBG 1
#define D(s,v) if(DBG) cout<<(s)<<": "<<(v)<<endl;
#define DVEC(s,v) if(DBG) {cout<<(s)<<": "; for(int i=0;i<(v).size();i++) cout<<(v)[i]<<" "; cout<<endl;}
#define DARR(s,v,n) if(DBG) {cout<<(s)<<": "; for(int i=0;i<n;i++) cout<<(v)[i]<<" "; cout<<endl;}


int main() {
ios_base::sync_with_stdio(0);

int n, k;
while (cin>>n>>k) {
    vector<int> a;
    for (int i = 0; i < n; i++) {
        int x; cin>>x;
        a.push_back(x);
    }

    vector<int> as = a;
    sort(as.begin(),as.end());

    int lo = 0, hi = k - 1;

    while (lo < hi) {
        int mid = (lo + hi) / 2;
        int x = as[mid];
        
        int pos = 0;
        for (int i = 0; i < n; i++) {
            if (pos % 2 == 0) {
                if (a[i] <= x) pos++;
            } else {
                pos++;
            }
        }

        if (pos >= k) {
            hi = mid;
            continue;
        }

        pos = 0;
        for (int i = 0; i < n; i++) {
            if (pos % 2 == 1) {
                if (a[i] <= x) pos++;
            } else {
                pos++;
            }
        }

        if (pos >= k) {
            hi = mid;
            continue;
        }

        lo = mid + 1;
    }

    cout<<as[lo]<<endl;
}

}