1 2  2 3  3 4  4 5  5 6  6 7  7 8  8 9  9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29 30 30 31 31 32 32 33 33 34 34 35 35 36 36 37 37 38 38 39 39 40 40 41 41 42 42 43 43 44 44 45 45 46 46 47 47 48 48 49 49 50 50 51 51 52 52 53 53 54 54 55 55 56 56 57 57 58 58 59 59 60 60 61 61 62 62 63 63 64 64 65 65 66 66 67 67 68 68 69 69 /************************************************************** 70     Problem: 1914 71     User: rausen 72     Language: C++ 73     Result: Accepted 74     Time:88 ms 75     Memory:3160 kb 76 ****************************************************************/ 77   78 #include <cstdio> 79 #include <cmath> 80 #include <algorithm> 81   82 using namespace std; 83 typedef long long ll; 84 typedef double lf; 85 const int N = 100005; 86   87 int n; 88 ll cnt = 0; 89   90 struct P { 91     ll x, y; 92     lf an; 93     P() {} 94     P(ll _x, ll _y, lf _an) : x(_x), y(_y), an(_an) {} 95 }a[N]; 96 inline bool operator < (const P &a, const P &b) { 97     return a.an < b.an; 98 } 99 inline ll operator * (const P &a, const P &b) {100     return (ll) a.x * b.y - a.y * b.x;101 }102  103 inline int read() {104     int x = 0, sgn = 1;105     char ch = getchar();106     while (ch < ‘0‘ || ‘9‘ < ch) {107         if (ch == ‘-‘) sgn = -1;108         ch = getchar();109     }110     while (‘0‘ <= ch && ch <= ‘9‘) {111         x = x * 10 + ch - ‘0‘;112         ch = getchar();113     }114     return sgn * x;115 }116  117 void work() {118     int r = 1, t = 0, i;119     for (i = 1; i <= n; ++i) {120         while ((r % n + 1) != i && a[i] * a[r % n + 1] >= 0) ++t, ++r;121         cnt += (ll) t * (t - 1) / 2;122         --t;123     }124 }125  126 int main() {127     n = read();128     int i, X, Y;129     for (i = 1; i <= n; ++i) {130         X = read(), Y = read();131         a[i] = P(X, Y, atan2(Y, X));132     }133     sort(a + 1, a + n + 1);134     work();135     printf("%lld\n", (ll) n * (n - 1) * (n - 2) / 6 - cnt);136     return 0;137 }

 1 /************************************************************** 2     Problem: 1914 3     User: rausen 4     Language: C++ 5     Result: Accepted 6     Time:60 ms 7     Memory:4728 kb 8 ****************************************************************/ 9  10 #include <cstdio>11 #include <cmath>12 #include <algorithm>13  14 using namespace std;15 typedef long long ll;16 typedef double lf;17 const int N = 100005;18 const int Maxbuf = 1600005;19 int n;20 ll cnt = 0, Left = 0, len;21 char buf[Maxbuf];22  23 struct P {24     ll x, y;25     lf an;26     P() {}27     P(ll _x, ll _y, lf _an) : x(_x), y(_y), an(_an) {}28 }a[N];29 inline bool operator < (const P &a, const P &b) {30     return a.an < b.an;31 }32 inline ll operator * (const P &a, const P &b) {33     return (ll) a.x * b.y - a.y * b.x;34 }35  36 inline int read() {37     int x = 0, sgn = 1;38     while (buf[Left] < ‘0‘ || ‘9‘ < buf[Left]) {39         if (buf[Left] == ‘-‘) sgn = -1;40         ++Left;41     }42     while (‘0‘ <= buf[Left] && buf[Left] <= ‘9‘) {43         x = x * 10 + buf[Left] - ‘0‘;44         ++Left;45     }46     return sgn * x;47 }48  49 void work() {50     int r = 1, t = 0, i;51     for (i = 1; i <= n; ++i) {52         while ((r % n + 1) != i && a[i] * a[r % n + 1] >= 0) ++t, ++r;53         cnt += (ll) t * (t - 1) / 2;54         --t;55     }56 }57  58 int main() {59     len = fread(buf, 1, Maxbuf, stdin);60     buf[len] = ‘ ‘;61     n = read();62     int i, X, Y;63     for (i = 1; i <= n; ++i) {64         X = read(), Y = read();65         a[i] = P(X, Y, atan2(Y, X));66     }67     sort(a + 1, a + n + 1);68     work();69     printf("%lld\n", (ll) n * (n - 1) * (n - 2) / 6 - cnt);70     return 0;71 }