poj3579(二分）

链接:https://cn.vjudge.net/problem/POJ-3579

对于中位数进行二分使其等于m

#include <cstdio>
#include <algorithm>
#include<iostream>
using namespace std;
long long n, a[100005];
int main()
{
    long long i, j;
    while(~scanf("%I64d", &n))
    {
        for(i = 1; i <= n; i ++)
            scanf("%I64d", a + i);
        std::sort(a + 1, a + n + 1);
        long long l = 0, r = a[n], mid, m = (n-1)*n/4, num;
        if((n*(n-1)/2)&1) m ++;
        //cout<<m<<endl;
        if(n == 1)
        {
            printf("0\n");
            continue;
        }
        while(l < r)
        {
            mid = (l + r) >> 1;
            j = 1;
            num = 0;
            for(i = 2; i <= n; i ++)
            {
                while(a[i] - a[j] > mid)
                    j ++;
                num += (i-j);
            }
            if(num >= m)
                l = mid;
            else r = mid + 1;
        }
        printf("%I64d\n", l);
    }
    return 0;
}