codeforces 659D . Bicycle Race 几何

题目链接

对相邻的三个点叉积判断一下就好。

#include <iostream>
#include <vector>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <complex>
#include <cmath>
#include <map>
#include <set>
#include <string>
#include <queue>
#include <stack>
#include <bitset>
using namespace std;
#define pb(x) push_back(x)
#define ll long long
#define mk(x, y) make_pair(x, y)
#define lson l, m, rt<<1
#define mem(a) memset(a, 0, sizeof(a))
#define rson m+1, r, rt<<1|1
#define mem1(a) memset(a, -1, sizeof(a))
#define mem2(a) memset(a, 0x3f, sizeof(a))
#define rep(i, n, a) for(int i = a; i<n; i++)
#define fi first
#define se second
typedef complex <double> cmx;
typedef pair<int, int> pll;
const double PI = acos(-1.0);
const double eps = 1e-8;
const int mod = 1e9+7;
const int inf = 1061109567;
const int dir[][2] = { {-1, 0}, {1, 0}, {0, -1}, {0, 1} };
const int maxn = 1005;
int cross(int x1, int y1, int x2, int y2, int x3, int y3) {
	return (x2 - x1) * (y3 - y1) - (y2 - y1) * (x3 - x1);
}
int x[maxn], y[maxn];
main()
{
    int n;
    cin>>n;
    n++;
    for(int i = 0; i < n; i++){
        cin>>x[i]>>y[i];
    }
    n--;
    int ans = 0;
    for(int i = 0; i < n; i++){
        ans += (cross(x[i], y[i], x[(i+1)%n], y[(i+1)%n], x[(i+2)%n], y[(i+2)%n])>0);
    }
    cout<<ans<<endl;
}