CodeForces

按坐标排完序后，选定前两个点，枚举后面的点

两种情况：

2. 如果与前两个点在一条直线上不用考虑，对后面的点不产生影响

1. 与前两个点不在一条直线上，直接输出三个点，就算第三个点之前有点在前两个点的直线上，也不影响边肯定在外面

#include<iostream>#include<algorithm>#include<cstdio>#include<cstdlib>#include<cstring>#include<string>#include<cmath>#include<set>#include<queue>#include<stack>#include<map>using namespace std;typedef long long ll;const int maxn = 1e5 + 7, INF = 0x7f7f7f7f, mod = 1e9 + 7;int n;struct node {    ll x, y;    int id;}a[maxn];bool cmp(node a, node b) {    return a.x <b.x || (a.x == b.x && a.y < b.y);}bool not_z(node a, node b, node c) {    if((b.y-a.y)*(c.x-b.x) == (c.y-b.y)*(b.x-a.x)) return false;    else return true;}int main() {    scanf("%d", &n);    for(int i = 1; i <= n; ++i) {        scanf("%lld %lld", &a[i].x, &a[i].y);        a[i].id = i;    }    sort(a+1, a+1+n, cmp);    for(int i = 2; i <= n; ++i) {        if(not_z(a[1], a[2], a[i])) {            printf("%d %d %d\n", a[1].id, a[2].id, a[i].id);            return 0;        }    }    return 0;}