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 + 5;
LL a[N], k, n;

bool judge(LL val) {
    LL cnt = 0;
    for(int i = 1; i <= n; ++i) {
        LL pos = (LL)(upper_bound(a + 1, a + n + 1, val - a[i]) - a);
        if(pos > (LL)i)
            cnt += n - pos + 1;
        else
            cnt += n - pos;
    }
    cnt /= 2;
    return (n - 1)*n/2 - k >= cnt;
}

int main()
{
    while(~scanf("%lld %lld", &n, &k)) {
        for(int i = 1; i <= n; ++i) {
            scanf("%lld", a + i);
        }
        sort(a + 1, a + n + 1);
        LL l = a[1] + a[2], r = a[n] + a[n - 1];
        while(l < r) {
            LL mid = (l + r) / 2;
            if(judge(mid)) {
                r = mid;
            } else {
                l = mid + 1;
            }
        }
        printf("%lld
", l);
    }
    return 0;
}