整数

using ll = long long;
ll l = -1,r = 1e18, lans, rans;
while(l < r) {
    ll lmid = l + (r - l) / 3; // l + 1/3区间大小
    ll rmid = r - (r - l) / 3;  // r - 1/3区间大小
    lans = cal(lmid),rans = cal(rmid);
    
    // 求凹函数的极小值
    if(lans <= rans) r = rmid - 1;
    else l = lmid + 1;
    
    // 求凸函数的极大值
    if(lans <= rans) l = lmid + 1;
    else r = rmid - 1;
}
// 求凹函数的极小值
cout << min(lans,rans) << endl;
// 求凸函数的极大值
cout << max(lans,rans) << endl;

浮点数

const double EPS = 1e-9;
double l = 0, r = 1e18, lans, rans;
while(r - l > EPS) {
    double lmid = l + (r - l) / 3;
    double rmid = r - (r - l) / 3;
    lans = cal(lmid),rans = cal(rmid);
    
    // 求凹函数的极小值
    if(lans <= rans) r = rmid;
    else l = lmid;
    
    // 求凸函数的极大值
    if(lans <= rans) l = lmid;
    else r = rmid;
}
// 输出 l 或 r 都可
cout << l << endl;