Gym 101064 D Black Hills golden jewels (二分)

题目链接：http://codeforces.com/gym/101064/problem/D

问你两个数组合相加的第k大数是多少。

先sort数组，二分答案，然后判断其正确性(判断过程是枚举每个数然后二分)。

 //#pragma comment(linker, "/STACK:102400000, 102400000")
 #include <algorithm>
 #include <iostream>
 #include <cstdlib>
 #include <cstring>
 #include <cstdio>
 #include <vector>
 #include <cmath>
 #include <ctime>
 #include <list>
 #include <set>
 #include <map>
 using namespace std;
 typedef long long LL;
 typedef pair <int, int> P;
 const int N = 1e5 + ;
 LL a[N], k, n;

 bool judge(LL val) {
     LL cnt = ;
     for(int i = ; i <= n; ++i) {
         LL pos = (LL)(upper_bound(a + , a + n + , val - a[i]) - a);
         if(pos > (LL)i)
             cnt += n - pos + ;
         else
             cnt += n - pos;
     }
     cnt /= ;
     return (n - )*n/ - k >= cnt;
 }

 int main()
 {
     while(~scanf("%lld %lld", &n, &k)) {
         for(int i = ; i <= n; ++i) {
             scanf("%lld", a + i);
         }
         sort(a + , a + n + );
         LL l = a[] + a[], r = a[n] + a[n - ];
         while(l < r) {
             LL mid = (l + r) / ;
             if(judge(mid)) {
                 r = mid;
             } else {
                 l = mid + ;
             }
         }
         printf("%lld\n", l);
     }
     return ;
 }