C - Ice Skating

Codeforces Round #134 (Div. 1)

Bajtek is learning to skate on ice. He's a beginner, so his only mode of transportation is pushing off from a snow drift to the north, east, south or west and sliding until he lands in another snow drift. He has noticed that in this way it's impossible to get from some snow drifts to some other by any sequence of moves. He now wants to heap up some additional snow drifts, so that he can get from any snow drift to any other one. He asked you to find the minimal number of snow drifts that need to be created.

We assume that Bajtek can only heap up snow drifts at integer coordinates.

Input
The first line of input contains a single integer n (1 ≤ n ≤ 100) — the number of snow drifts. Each of the following n lines contains two integers xi and yi (1 ≤ xi, yi ≤ 1000) — the coordinates of the i-th snow drift.

Note that the north direction coinсides with the direction of Oy axis, so the east direction coinсides with the direction of the Ox axis. All snow drift's locations are distinct.

Output
Output the minimal number of snow drifts that need to be created in order for Bajtek to be able to reach any snow drift from any other one.

Example
Input
2
2 1
1 2
Output
1
Input
2
2 1
4 1
Output
0

题意：二维坐标系，有很多雪堆，你只能平行于x轴或y轴从一个雪堆移动到另个个雪堆处，问最少添加几个雪堆，使你能到达全部雪堆

解法：bfs，类似油田问题，能互相到达的一些雪堆为一个块，判断有几块雪堆，减一即为要添加的雪堆个数。

代码：

#include<iostream>
#include<cstring>
#include<queue>
using namespace std;
int n;
int x[1005];
int y[1005];
bool vis[1005];
queue<int> q;
int ans=0;
void bfs()
{
    while(!q.empty()){
        int t=q.front();
        q.pop();
        vis[t]=1;
        for(int i=0;i<n;i++)
            if(!vis[i]&&(x[i]==x[t]||y[i]==y[t]))
                q.push(i);
    }
}
int main()
{
    cin>>n;
    memset(vis,0,sizeof(vis));
    for(int i=0;i<n;i++)
        cin>>x[i]>>y[i];
    for(int i=0;i<n;i++)
        if(!vis[i]){
            q.push(i);
            bfs();
            ans++;
        }
    cout<<ans-1<<endl;
}