HDU 1392 (convex bag)

Portal: Surround the Trees

Test instructions: To find the perimeter of the convex hull.

Analysis: Convex hull template problem, first ordered by the pole angle, and then determine the convex hull according to the cross product positive and negative.

#include <stdio.h>#include<math.h>#include<algorithm>#include<string.h>#include<math.h>using namespacestd;Const DoubleEPS = 1e-8;Const intN = the;intSgnDoublex) {    if(Fabs (x) < EPS)return 0; if(X <0)return-1; Else return 1;}structpoint{Doublex, y; Point () {}, point (Double_x,Double_y) {x= _x;y =_y; } Pointoperator-(ConstPoint &b)Const    {        returnPoint (X-b.x,y-b.y); }    //Cross Product    Double operator^(ConstPoint &b)Const    {        returnx*b.y-y*b.x; }    //dot Product    Double operator*(ConstPoint &b)Const    {        returnx*b.x + y*b.y; }};DoubleDist (Point A,point b) {returnsqrt (A-B) * (Ab));} Point P[n];intStack[n],top;BOOLCMP (Point p1,point p2) {DoubleTmp= (p1-p[0]) ^ (p2-p[0]); if(SGN (tmp) = =0)returnSGN (Dist (p[0],P1)-dist (p[0],P2)) <=0; returnSGN (TMP) >0;}voidGraham (intN) {Sort (P+1, p+n,cmp); stack[0]=0; stack[1]=1; Top=2;  for(intI=2; i<n;i++)    {         while(top>1&AMP;&AMP;SGN ((p[stack[top-1]]-p[stack[top-2]]) ^ (p[i]-p[stack[top-2]]) <=0) top--; Stack[top++]=i; }}intMain () {intN;  while(SCANF ("%d",&N), N) { for(intI=0; i<n;i++) {scanf ("%LF%LF",&p[i].x,&p[i].y); if(p[i].y<p[0].y| | p[i].y==p[0].y&&p[i].x<p[0].x) Swap (p[0],p[i]); }        if(n==1) puts ("0.00"); Else if(n==2) printf ("%.2lf\n", Dist (p[0],p[1])); Else{Graham (n); Doubleans=0;  for(intI=0; i<top;i++) ans+=dist (p[stack[i]],p[stack[(i+1)%top]]); printf ("%.2lf\n", ans); }    }    return 0;}

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HDU 1392 (convex bag)