二分答案,lowerbound

#include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
const int maxn = (int) 1e5 + 7;
int A[maxn];
int binT[maxn];
int m;

bool check(int x,int n)
{
int cnt = 0;
for (int i = 0; i<n - 1; i++)
{
cnt += upper_bound(A + i + 1, A + n, x + A[i]) - (A + i + 1);
}
if (cnt >= m)
return true;
else
return false;
}
int main()
{
int n;
while (cin >> n)
{
for (int i = 0; i<n; i++)
{
scanf("%d", &A[i]);
}
sort(A, A + n);
int d = (n*(n - 1) / 2);
if ((d % 2)==0)
{
m = (d / 2);
}
else
{
m = (d + 1) / 2;
}
int left = 0, right = A[n - 1] - A[0] + 1;
while (right>left)
{
int mid = (left + right) >> 1;
if (check(mid,n))
right = mid;
else
left = mid + 1;
}
cout << left << endl;
}
return 0;
}