UVALive 6168 Fat Ninjas --二分小数+搜索

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <string>
#include <vector>
using namespace std;
#define N 5007

struct node
{
    int x,y;
}p[N];

double d[N][N];
vector<int> G[N];
int n,L;
int vis[N],S,E;

double dis(node ka,node kb)
{
    return (double)sqrt((ka.x-kb.x)*(ka.x-kb.x)+(ka.y-kb.y)*(ka.y-kb.y));
}

int dfs(int u,int fa)
{
    if(u == E)
        return 1;
    vis[u] = 1;
    for(int i=0;i<G[u].size();i++)
    {
        int v = G[u][i];
        if(v == fa)
            continue;
        if(vis[v])
            continue;
        if(dfs(v,u))
            return 1;
    }
    return 0;
}

bool check(double D)
{
    S = 0;
    E = L+1;
    for(int i=0;i<=L+1;i++)
        G[i].clear();
    for(int i=1;i<=L;i++)
    {
        if(p[i].y < D)
            G[S].push_back(i);
        if(p[i].y + D > n)
            G[i].push_back(E);
        for(int j=i+1;j<=L;j++)
        {
            if(d[i][j] < D)
            {
                G[i].push_back(j);
                G[j].push_back(i);
            }
        }
    }
    memset(vis,0,sizeof(vis));
    if(dfs(S,-1))
        return 0;
    return 1;
}

int main()
{
    int i,j;
    while(scanf("%d%d",&n,&L)!=EOF && n)
    {
        for(i=1;i<=L;i++)
            scanf("%d%d",&p[i].x,&p[i].y);
        for(i=1;i<=L;i++)
            for(j=i+1;j<=L;j++)
                d[i][j] = dis(p[i],p[j]);
        double low = eps;
        double high = n;
        while(low+eps < high)
        {
            double mid = (low+high)/2.0;
            if(check(mid))
                low = mid;
            else
                high = mid;
        }
        printf("%.3lf\n",low);
    }
    return 0;
}