复制代码 代码如下:
以HDU2108为例，去AC吧。
//点逆序输入
import java.util.Scanner;
//1s
public class HDU2108 {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
while(true) {
int x,y;
int n = sc.nextInt();
if(0==n) {
break;
}
Point[] p = new Point[n];
for(int i=0; i<n; i++) {
p[i] = new Point();
}
for(int i=0; i<n; i++) {
x = sc.nextInt();
y = sc.nextInt();
p[i] = new Point(x,y);
}
//凸多边形
boolean tag = true;
int j,k,t;
for(int i=0; i<n; i++) {
//k,t直接对n求余就行了
j = i;
k = i+1;
t = i+2;
//以三角形为例看看
if(k==n) {
k = 0;
}
if(t==n+1) {
t = 1;
}
if(t==n) {
t = 0;
}
//注意是后面减去前面的
Point p1 = new Point(p[k].x - p[j].x,
p[k].y - p[j].y);
Point p2 = new Point(p[t].x - p[k].x,
p[t].y - p[k].y);
//叉积
int ans = p1.x*p2.y - p1.y*p2.x;
if(ans<0) {
tag = false;
break;
}
}
if(tag) {
System.out.println("convex");
}else {
System.out.println("concave");
}
}
}
}
class Point {
int x;
int y;
public Point() {
this.x = 0;
this.y = 0;
}
public Point(int x, int y) {
this.x = x;
this.y = y;
}
}
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