CF 1370D 二分 2e5

 1 #include<bits/stdc++.h>
 2 #define ll long long
 3 #define INF 1e17
 4 using namespace std;
 5 const int N = 2e5 + 10; 
 6 ll n, k;
 7 ll a[N];
 8 
 9 bool check(ll mid, ll cur)
10 {
11     int len = 0;
12     for(int i = 1 ; i <= n ; i++){
13         if(cur){
14             if(a[i] <= mid){
15                 len++;
16                 cur ^= 1;
17             }
18         }else{
19             len++;
20             cur ^= 1;
21         }
22     }
23     return len >= k;
24 }
25 
26 int main(){
27     scanf("%lld%lld",&n,&k);
28     for(int i = 1 ; i <= n ; i++){
29         scanf("%lld",&a[i]);
30     }
31     ll l = 1, r = INF;
32     while(l < r)
33     {
34         ll mid = (l + r) >> 1;
35         if(check(mid, 0) || check(mid, 1)){
36             r = mid;
37         }else{
38             l = mid + 1;
39         }
40     }
41     printf("%lld\n",l);
42     
43     return 0;
44 }