Poj 2560 freckles Minimum Spanning Tree

I haven't written any questions about the minimal spanning tree for a long time. I came across a question about the minimal spanning tree. I didn't expect 1a to happen. It uses the Kruskal algorithm, which is relatively simple. Question:

Freckles

Time limit:1000 ms		Memory limit:65536 K
Total submissions:5046		Accepted:2630

Description

In an episode of The Dick Van Dyke Show, little Richie connects the freckles on his dad's back to form a picture of the Liberty Bell. alas, one of the freckles turns out to be a scar, so his Ripley's engagement falls through.
Consider Dick's back to be a plane with freckles at various (x, y) locations. your job is to tell Richie how to connect the dots so as to minimize the amount of ink used. richie connects the dots by drawing straight lines between pairs, possibly lifting
Pen between lines. When Richie is done there must be a sequence of connected lines from any freckle to any other freckle.

Input

The first line contains 0 <n <= 100, the number of freckles on Dick's back. for each freckle, a line follows; each following line contains two real numbers indicating the (x, y) coordinates of the freckle.

Output

Your program prints a single real number to two decimal places: the minimum total length of ink lines that can connect all the freckles.

Sample Input

31.0 1.02.0 2.02.0 4.0

Sample output

3.41

AC code:

#include <iostream>#include <string.h>#include <cmath>#include <algorithm>using namespace std;const int N=110;int father[N];int n,k;struct point{double x,y;}pp[N];struct edge{int lp,rp;double len;}ee[N*N];double fun(double x){return x*x;}void init(){for(int i=1;i<=n;++i)father[i]=i;k=0;for(int i=1;i<=n;++i){for(int j=1;j<=n;++j){  if(i==j)continue;  ee[k].lp=i;  ee[k].rp=j;  ee[k++].len=sqrt( fun(pp[i].x-pp[j].x)+fun(pp[i].y-pp[j].y) );}}}bool cmp(edge a,edge b){return a.len<b.len;}int find(int x){if(x!=father[x])father[x]=find(father[x]);return father[x];}bool Union_Set(int x,int y){int rootx=find(x);int rooty=find(y);if(rootx!=rooty){  father[rooty]=rootx;  return true;}return false;}int main(){//freopen("1.txt","r",stdin);while(~scanf("%d",&n)){  for(int i=1;i<=n;++i)  scanf("%lf%lf",&pp[i].x,&pp[i].y);  init();  sort(ee,ee+k,cmp);  double sum=0.0;  for(int i=0;i<k;++i){  int x=ee[i].lp;  int y=ee[i].rp;  bool flag=Union_Set(x,y);  if(flag)sum+=ee[i].len;  }  printf("%.2lf\n",sum);}return 0;}