三分探索

ternary_search.cpp

double l = 0, r = 1000;
for (int i = 0; i < 80; ++i) {
  double c1, c2;
  c1 = (l * 2 + r) / 3;
  c2 = (l + r * 2) / 3;
  if (f(c1) > f(c2)) {
    l = c1;
  } else {
    r = c2;
  }
}