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source/Geometry/Geo.cpp

@@ -0,0 +1,26 @@
+forr(i, now) vec.pb(A[i]);
+sort(all(vec));
+
+vll upper;
+for(auto i:vec)
+{
+    while(upper.size() >= 2 && ccw(upper[upper.size()-2], upper.back(), i) >= 0)
+        upper.pop_back();
+    upper.push_back(i);
+}
+
+reverse(all(vec));
+vll lower;
+for(auto i:vec)
+{
+    while(lower.size() >= 2 && ccw(lower[lower.size()-2], lower.back(), i) >= 0)
+        lower.pop_back();
+    lower.push_back(i);
+}
+
+assert(upper.front() == lower.back() && lower.front() == upper.back());
+upper.pop_back(); lower.pop_back();
+
+vll hull;
+hull.insert(hull.end(), all(upper));
+hull.insert(hull.end(), all(lower));