poj2236 Wireless Network(并查集)

https://vjudge.net/problem/POJ-2236

快4s过的，但是排行榜上很多1s内，不知道如何优化。。

#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<stack>
#define lson l, m, rt<<1
#define rson m+1, r, rt<<1|1
#define IO ios::sync_with_stdio(false);cin.tie(0);
#define INF 0x3f3f3f3f
typedef unsigned long long ll;
using namespace std;
int n, num, d, s, t, pre[10010];
char c;
typedef struct{
    int x, y;
}Node;
Node node[10010];
int flag[10010];
ll length(ll a, ll b)
{
    return a*a+b*b;
}
int find(int x)
{
    while(x != pre[x]){
        x = pre[x];
    }
    return x;
}
void join(int k)
{
    int tx = find(k); 
    pre[k] = tx;//优化 
    for(int i = 1; i <= n; i++){
        if(flag[i]&&i != k){
            ll tmp = length(node[k].x-node[i].x, node[k].y-node[i].y);
            if(tmp<=d*d){//一开始直接pre[i]=tx然后wa了，一定要是把根接上去 
                int ty = find(i);
                pre[i] = ty;//优化 
                if(tx != ty){
                    pre[ty] = tx;
                }
            }
        }
    }
}
int main()
{
    IO;
    memset(flag, 0, sizeof(flag));
    cin >> n >> d;
    for(int i = 1; i <= n; i++){
        pre[i] = i;
    }
    for(int i = 1; i <= n; i++){
        cin >> node[i].x >> node[i].y;
    } 
    while(cin >> c){
        if(c == 'O'){
            cin >> num;
            flag[num] = 1;
            join(num);
        }
        else if(c == 'S'){
            cin >> s >> t;
            int tx = find(s);
            pre[s] = tx; 
            int ty = find(t);
            pre[t] = ty;
            if(tx == ty) cout << "SUCCESS" << endl;
            else cout << "FAIL" << endl;
        }
    }
    return 0; 
}