Created
June 29, 2021 00:07
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//Código por Pedro Bignotto Racchetti | |
//O(nlogn) | |
#include<bits/stdc++.h> | |
using namespace std; | |
struct Pt { | |
int x, y; | |
Pt(int _x = 0, int _y = 0) : x(_x), y(_y) {} | |
Pt operator+(const Pt& o) { return Pt(x+o.x, y+o.y); } // soma de vetores | |
Pt operator-(const Pt& o) { return Pt(x-o.x, y-o.y); } // subtração de vetores | |
bool operator < (Pt o) const{ // ordena pelas abscissas e depois pelas ordenadas | |
if(x == o.x) return y < o.y; | |
return x < o.x; | |
} | |
void operator+=(const Pt& o) { x += o.x, y += o.y; } // somar vetores com o operador += | |
void operator-=(const Pt& o) { x -= o.x, y -= o.y; } // subtrair vetores com o operador -= | |
long long dot(const Pt& o) { return 1ll*x*o.x + 1ll*y*o.y; } // dot - produto escalar | |
long long cross(const Pt& o) { return 1ll*x*o.y - 1ll*y*o.x; } // cross - produto vetorial | |
}; | |
Pt pts[112345]; | |
int marc[112345]; | |
int n; | |
vector<Pt> conv, upper, lower; | |
void makeconv(){ | |
long long it1 = 0, it2 = 0; | |
for(int i = 0; i < n; i++){ | |
if(marc[i]) continue; | |
while(it1 >= 2 && (pts[i] - upper[it1 - 2]).cross(upper[it1 - 1] - upper[it1 -2]) < 0){ | |
upper.pop_back(); | |
it1--; | |
} | |
upper.push_back(pts[i]); | |
it1++; | |
while(it2 >= 2 && (pts[i] - lower[it2 - 2]).cross(lower[it2 - 1] - lower[it2 -2]) > 0){ | |
lower.pop_back(); | |
it2--; | |
} | |
lower.push_back(pts[i]); | |
it2++; | |
} | |
for(int i = 0; i < it1 - 1; i++) conv.push_back(upper[i]); | |
for(int i = it2 - 1; i > 0; i--) conv.push_back(lower[i]); | |
} | |
int main(){ | |
scanf("%d", &n); | |
for(int i = 0; i < n; i++){ | |
scanf("%lld %lld", &pts[i].x, &pts[i].y); | |
} | |
sort(pts, pts + n); | |
makeconv(); | |
printf("%lu\n", conv.size()); | |
for(int i = 0; i < conv.size(); i++ ){ | |
printf("%lld %lld\n", conv[i].x, conv[i].y); | |
} | |
scanf("%d", &n); | |
} |
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