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  • 判断平面的一堆点是否在两条直线上

    D. Pair Of Lines
    time limit per test
    2 seconds
    memory limit per test
    256 megabytes
    input
    standard input
    output
    standard output

    You are given n points on Cartesian plane. Every point is a lattice point (i. e. both of its coordinates are integers), and all points are distinct.

    You may draw two straight lines (not necessarily distinct). Is it possible to do this in such a way that every point lies on at least one of these lines?

    Input

    The first line contains one integer n (1 ≤ n ≤ 105) — the number of points you are given.

    Then n lines follow, each line containing two integers xi and yi (|xi|, |yi| ≤ 109)— coordinates of i-th point. All n points are distinct.

    Output

    If it is possible to draw two straight lines in such a way that each of given points belongs to at least one of these lines, print YES. Otherwise, print NO.

    Examples
    Input
    Copy
    5
    0 0
    0 1
    1 1
    1 -1
    2 2
    Output
    YES
    Input
    Copy
    5
    0 0
    1 0
    2 1
    1 1
    2 3
    Output
    NO
    Note

    In the first example it is possible to draw two lines, the one containing the points 1, 3 and 5, and another one containing two remaining points.

    题意 : 给定平面的一些点,问是否最多用两条直线去描绘出这些点。

    思路分析 : 对于平面一堆点中的任意3个,若其可以用两条直线去表示,则一定 1 3 或者 2 3 或者 1 2 是在一起的。那么就根据这个去判断每个点就行。

    注意斜率 ! 判断两个斜率是否相等的时候需要用乘的关系去判断,除的关系去判断时若斜率不存在时,则会出现问题,分母为 0 了

    注意 long long

    代码示例 :

    #define ll long long
    const ll maxn = 1e5+5;
    const double pi = acos(-1.0);
    const ll inf = 0x3f3f3f3f;
    
    ll n;
    struct node
    {
        ll x, y;
    }pre[maxn];
    bool vis[maxn];
    
    bool fun(node a, node b, node c){
        ll k1 = (b.y-a.y)*(c.x-a.x);
        ll k2 = (c.y-a.y)*(b.x-a.x);
        return k1==k2?true:false;
    }
    
    bool check(ll a, ll b){
        
        memset(vis, false, sizeof(vis));
        vis[a] = vis[b] = true;
        
        for(ll i = 1; i <= n; i++){
            if (i == a || i == b) continue;
            if (fun(pre[a], pre[b], pre[i])) vis[i] = true;
        }
        ll p1=-1, p2=-1;
        for(ll i = 1; i <= n; i++){
            if (!vis[i]){
                if(p1 == -1) {p1=i;}
                else if (p2==-1){p2=i;}
            }
        }
        if (p1==-1||p2==-1) return true;
        for(ll i = 1; i <= n; i++){
            if (i == p1 || i == p2) continue;
            if (!vis[i] && !fun(pre[p1], pre[p2], pre[i])) return false;
        }
        return true;
    }
    
    int main() {
        //freopen("in.txt", "r", stdin);
        //freopen("out.txt", "w", stdout);
        cin >> n;
        for(ll i = 1; i <= n; i++){
            scanf("%lld%lld", &pre[i].x, &pre[i].y);        
        }
        if (n < 5 || check(1, 2) || check(1, 3) || check(2, 3)){
            printf("YES
    ");
        }
        else printf("NO
    ");
        return 0;
    }
    
    东北日出西边雨 道是无情却有情
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  • 原文地址:https://www.cnblogs.com/ccut-ry/p/8734329.html
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